Magnesium sulfate (Epsom salt)

Also called Epsom salt, Epsom salts, magnesium sulphate, MgSO4, magnesium sulfate heptahydrate. One of 9 forms of magnesium compared on this page.

Grade B for sulfate itself: functional constipation in women (mineral water) Mineral Last checked

The short answer

From 25 sources

Magnesium sulfate is Epsom salt: about 10% magnesium by weight and an over-the-counter saline laxative. Mineral waters rich in magnesium sulfate eased functional constipation in several trials (grade B). No controlled study shows that Epsom baths raise body magnesium. Swallowed in large amounts it can cause dangerous magnesium levels.

Every graded outcome on this page

7 rows · line = participants
  1. Grade BFunctional constipation in women (mineral water)Tested sulfaten 244
  2. Grade BTreatment response in functional constipation (mineral water)Tested sulfaten 226
  3. Grade BBowel movements and stool consistency in functional constipationTested sulfaten 75
  4. Grade CBowel movement frequency in functional constipationTested sulfaten 100
  5. Grade DArterial stiffness and blood pressure (daily supplement)Tested sulfaten 164
  6. Grade DGut transit speed and stool weight (oral laxative dose)Tested sulfaten 8
  7. No effectNight-time leg cramps in older adults (magnesium in any form)Other or mixed formsn 307
  8. 1 more graded outcome, in the evidence chapter
Tested sulfateOther or mixed forms6 of 7 tested sulfate itself.

Key facts

Elemental magnesium
About 9.9% of Epsom salt (the heptahydrate) is magnesium: a 5 g teaspoon holds 495 mg; anhydrous magnesium sulfate is about 20% magnesium123
What the label is for
US Epsom salt is an OTC saline laxative for occasional constipation that generally works in 30 minutes to 6 hours; many packs also list a soak for minor sprains and bruises14
Laxative dose vs upper limit
Adult label directions of 10 to 30 g a day give 990 to 2,970 mg of magnesium, several times the 350 mg/day upper limit for supplemental magnesium, which is why the label is for occasional, short use15
Strongest human evidence
Functional constipation (B): 1 L a day of a magnesium sulfate-rich mineral water helped more women than low-mineral water within 2 weeks in two French trials (244 and 226 people)67
Baths and soaks
A 2017 review concluded that claims for transdermal magnesium, salt baths included, are scientifically unsupported11
As a daily supplement
450 mg/day of magnesium as sulfate for 24 weeks raised blood magnesium about as much as citrate or oxide but did not change arterial stiffness or blood pressure10
Overdose
Case reports describe fatal and near-fatal high blood magnesium after swallowing Epsom salt, even with normal kidneys1213
Hospital use
As a prescription injection it replaces magnesium and controls seizures in pre-eclampsia and eclampsia; in the 10,141-woman Magpie trial it halved eclampsia1416

What it is

Magnesium sulfate is the magnesium salt sold as Epsom salt, an over-the-counter saline laxative and bath soak; also a hospital injection.

How it works

Established

Taken by mouth, magnesium sulfate acts as an osmotic laxative: it keeps water in the large bowel, which speeds transit and improves stool consistency; in volunteers, an oral dose accelerated small-bowel transit and raised stool weight. Magnesium that is absorbed is cleared only by the kidneys, so blood levels can climb when large amounts are swallowed or kidney function is poor. Injected magnesium blocks neuromuscular transmission and reduces acetylcholine release at the motor end-plate, the basis of its use against eclamptic seizures.1781412

Compound weight to elemental magnesium

Arithmetic, not advice

50mg elemental magnesium500 mg epsom salt crystals (magnesium sulfate heptahydrate, usp) × 9.9%

Arithmetic only, from the elemental fractions in the forms table. US Supplement Facts panels list the elemental amount; front-of-pack claims sometimes give the compound weight. Real products vary with hydration state and buffering.

Forms side by side: Epsom crystals, soaks, mineral water, capsules and injection

9 forms named in our sources, each with a page of its own. Citation numbers in a column link to the page that holds them.

Magnesium forms compared

9 forms · 9 with a page · Sulfate (Epsom salt) highlighted
Magnesium forms compared: 9 forms by 6 attributes. Magnesium sulfate (Epsom salt) is this page. Empty cells are labelled No data yet, or Not graded in the evidence row.
9 formsSulfate (Epsom salt)You are hereGlycinate (bisglycinate)Read the pageCitrateRead the pageL-threonateRead the pageMalateRead the pageTaurateRead the pageOxideRead the pageChlorideRead the pageOrotateRead the page
Elemental shareOf compound weight9.9%12MgSO4·7H2O: 24.3 ÷ 246.5 = 9.9% magnesium; the label gives 495 mg magnesium per 5 g teaspoon14.1%10Mg(C2H4NO2)2, anhydrous MW 172.4216.2%12Mg3(C6H5O7)2 (trimagnesium dicitrate), anhydrous MW 451.128.3%5Mg(C4H7O5)2, anhydrous MW 294.5015.5%19MgC4H4O5, anhydrous MW 156.388.9%20Mg(C2H6NO3S)2, anhydrous MW 272.660.3%6MgO, MW 40.30512%213Magnesium chloride hexahydrate MgCl2·6H2O, MW 203.30 (anhydrous MgCl2, MW 95.21, is 25.5% magnesium)6.6%26Dihydrate C10H10MgN4O10, MW 370.51 (6.6% Mg); anhydrous C10H6MgN4O8, MW 334.48 (7.3% Mg); Mg 24.305 g/mol
Form-specific evidenceOutcomes tested on this form
Grade BFunctional constipation in women (mineral water)Tested this formRCT, n 244, 2014
Grade BTreatment response in functional constipation (mineral water)Tested this formRCT, n 226, 2019
+4 more on its page
Grade CInflammation markers and lung function in asthma and COPDTested this formRCT, n 224, 2026
Grade CInsomnia severityTested this formRCT, n 155, 2025
+1 more on its page
Grade CArterial stiffness and blood pressureTested this formRCT, n 164, 2022
Grade CMigraine attack frequencyTested this formRCT, n 81, 1996
+4 more on its page
Grade COverall cognitive performanceTested this formRCT, n 100, 2025
Grade COpioid dose needed for pain in advanced cancerTested this formRCT, n 83, 2023
+2 more on its page		No form-specific graded outcome yetNo form-specific graded outcome yet
Grade BOverall improvement in chronic constipationTested this formRCT, n 90, 2021
Grade BOverall improvement in chronic constipationTested this formRCT, n 34, 2019
+2 more on its page
Grade CMetabolic syndrome and its components in adults with low magnesiumTested this formRCT, n 198, 2018
Grade CMild-to-moderate depression and anxiety symptomsTested this formCrossover RCT, n 126, 2017
+10 more on its page
Grade CHeart-rhythm, blood-pressure and related problemsTested this formMeta-analysis, n 1,190, 2015
Grade CSurvival and symptoms in severe heart failureTested this formRCT, n 79, 2009
+2 more on its page
Studied dosesRange in its trialsNo data yet100–250mgelemental magnesium (as glycinate/bisglycinate)342827290–600mgelemental magnesium (as citrate)6152221231083–165mgelemental magnesium (1–2 g/day of magnesium L-threonate compound)534192250–600mgmg of elemental magnesium per day (with malic acid)361121mgmg of elemental magnesium per day3500–1,500mgmg of magnesium oxide (compound) per day834512248–450mgElemental magnesium per day as stated in the oral trials134814151,878–6,000mgmg of magnesium orotate compound per day (not elemental magnesium)347
Absorption in peopleActs mainly in the bowel as an osmotic laxative: it keeps water in the large bowel and speeds transit.178Similar to oxide overall in 12 ileal-resection patients (23.5% vs 22.8%); higher in the 4 poorest oxide absorbers.11Beat oxide on urinary and serum magnesium in short human studies.1314No human head-to-head data; rat data only.16No human comparison; highest serum AUC of five compounds in rats.22No human data; fast absorption and highest brain levels in rats (acetyl taurate).22About 4% fractional absorption in one comparison, the lowest tested.2113ODS: small studies find chloride, citrate, aspartate and lactate absorbed more completely than oxide or sulfate.1No head-to-head human absorption data in our sources.1
GI toleranceWorks as a laxative within 30 minutes to 6 hours; the label warns against use with kidney disease, a magnesium-restricted diet or abdominal pain, nausea or vomiting.1All 12 crossover patients tolerated it better than oxide.11Diarrhea in 18.6% at 600 mg/day (12-week migraine trial).15Well tolerated at 1–2 g/day for 3–6 weeks.1718No form-specific human data.No serious adverse events in one 3-month study (taurate with potassium citrate).23Among the forms ODS links most often to diarrhea.1ODS lists chloride among the forms most commonly reported to cause diarrhea at high doses.1Large magnesium doses from supplements commonly cause diarrhoea, nausea and abdominal cramping.1
Studied forOccasional constipation (OTC label); transit studies (D)Insomnia severity (one RCT).Arterial stiffness (no effect), bone turnover, migraine.Cognition and sleep (small RCTs).Fibromyalgia (one small trial).Blood pressure (one uncontrolled study).Constipation, migraine; arterial stiffness null.Glucose, insulin resistance, blood pressure and CRP in adults with low magnesium; depression symptoms; heart rhythm in heart failure.Heart failure, heart rhythm and blood pressure (Russian trials)
  1. Magnesium sulfate (Epsom salt)You are here
    Elemental share
    9.9%12MgSO4·7H2O: 24.3 ÷ 246.5 = 9.9% magnesium; the label gives 495 mg magnesium per 5 g teaspoon
    Form-specific evidence
    Grade BFunctional constipation in women (mineral water)Tested this formRCT, n 244, 2014
    Grade BTreatment response in functional constipation (mineral water)Tested this formRCT, n 226, 2019
    +4 more on its page
    Studied doses
    No data yet
    Absorption in people
    Acts mainly in the bowel as an osmotic laxative: it keeps water in the large bowel and speeds transit.178
    GI tolerance
    Works as a laxative within 30 minutes to 6 hours; the label warns against use with kidney disease, a magnesium-restricted diet or abdominal pain, nausea or vomiting.1
    Studied for
    Occasional constipation (OTC label); transit studies (D)
  2. Magnesium glycinate (bisglycinate)Read the page
    Elemental share
    14.1%10Mg(C2H4NO2)2, anhydrous MW 172.42
    Form-specific evidence
    Grade CInflammation markers and lung function in asthma and COPDTested this formRCT, n 224, 2026
    Grade CInsomnia severityTested this formRCT, n 155, 2025
    +1 more on its page
    Studied doses
    100–250mgelemental magnesium (as glycinate/bisglycinate)342827
    Absorption in people
    Similar to oxide overall in 12 ileal-resection patients (23.5% vs 22.8%); higher in the 4 poorest oxide absorbers.11
    GI tolerance
    All 12 crossover patients tolerated it better than oxide.11
    Studied for
    Insomnia severity (one RCT).
  3. Magnesium citrateRead the page
    Elemental share
    16.2%12Mg3(C6H5O7)2 (trimagnesium dicitrate), anhydrous MW 451.12
    Form-specific evidence
    Grade CArterial stiffness and blood pressureTested this formRCT, n 164, 2022
    Grade CMigraine attack frequencyTested this formRCT, n 81, 1996
    +4 more on its page
    Studied doses
    290–600mgelemental magnesium (as citrate)61522212310
    Absorption in people
    Beat oxide on urinary and serum magnesium in short human studies.1314
    GI tolerance
    Diarrhea in 18.6% at 600 mg/day (12-week migraine trial).15
    Studied for
    Arterial stiffness (no effect), bone turnover, migraine.
  4. Magnesium L-threonateRead the page
    Elemental share
    8.3%5Mg(C4H7O5)2, anhydrous MW 294.50
    Form-specific evidence
    Grade COverall cognitive performanceTested this formRCT, n 100, 2025
    Grade COpioid dose needed for pain in advanced cancerTested this formRCT, n 83, 2023
    +2 more on its page
    Studied doses
    83–165mgelemental magnesium (1–2 g/day of magnesium L-threonate compound)534192
    Absorption in people
    No human head-to-head data; rat data only.16
    GI tolerance
    Well tolerated at 1–2 g/day for 3–6 weeks.1718
    Studied for
    Cognition and sleep (small RCTs).
  5. Magnesium malateRead the page
    Elemental share
    15.5%19MgC4H4O5, anhydrous MW 156.38
    Form-specific evidence
    No form-specific graded outcome yet
    Studied doses
    250–600mgmg of elemental magnesium per day (with malic acid)361
    Absorption in people
    No human comparison; highest serum AUC of five compounds in rats.22
    GI tolerance
    No form-specific human data.
    Studied for
    Fibromyalgia (one small trial).
  6. Magnesium taurateRead the page
    Elemental share
    8.9%20Mg(C2H6NO3S)2, anhydrous MW 272.6
    Form-specific evidence
    No form-specific graded outcome yet
    Studied doses
    121mgmg of elemental magnesium per day3
    Absorption in people
    No human data; fast absorption and highest brain levels in rats (acetyl taurate).22
    GI tolerance
    No serious adverse events in one 3-month study (taurate with potassium citrate).23
    Studied for
    Blood pressure (one uncontrolled study).
  7. Magnesium oxideRead the page
    Elemental share
    60.3%6MgO, MW 40.305
    Form-specific evidence
    Grade BOverall improvement in chronic constipationTested this formRCT, n 90, 2021
    Grade BOverall improvement in chronic constipationTested this formRCT, n 34, 2019
    +2 more on its page
    Studied doses
    500–1,500mgmg of magnesium oxide (compound) per day834512
    Absorption in people
    About 4% fractional absorption in one comparison, the lowest tested.2113
    GI tolerance
    Among the forms ODS links most often to diarrhea.1
    Studied for
    Constipation, migraine; arterial stiffness null.
  8. Magnesium chlorideRead the page
    Elemental share
    12%213Magnesium chloride hexahydrate MgCl2·6H2O, MW 203.30 (anhydrous MgCl2, MW 95.21, is 25.5% magnesium)
    Form-specific evidence
    Grade CMetabolic syndrome and its components in adults with low magnesiumTested this formRCT, n 198, 2018
    Grade CMild-to-moderate depression and anxiety symptomsTested this formCrossover RCT, n 126, 2017
    +10 more on its page
    Studied doses
    248–450mgElemental magnesium per day as stated in the oral trials13481415
    Absorption in people
    ODS: small studies find chloride, citrate, aspartate and lactate absorbed more completely than oxide or sulfate.1
    GI tolerance
    ODS lists chloride among the forms most commonly reported to cause diarrhea at high doses.1
    Studied for
    Glucose, insulin resistance, blood pressure and CRP in adults with low magnesium; depression symptoms; heart rhythm in heart failure.
  9. Magnesium orotateRead the page
    Elemental share
    6.6%26Dihydrate C10H10MgN4O10, MW 370.51 (6.6% Mg); anhydrous C10H6MgN4O8, MW 334.48 (7.3% Mg); Mg 24.305 g/mol
    Form-specific evidence
    Grade CHeart-rhythm, blood-pressure and related problemsTested this formMeta-analysis, n 1,190, 2015
    Grade CSurvival and symptoms in severe heart failureTested this formRCT, n 79, 2009
    +2 more on its page
    Studied doses
    1,878–6,000mgmg of magnesium orotate compound per day (not elemental magnesium)347
    Absorption in people
    No head-to-head human absorption data in our sources.1
    GI tolerance
    Large magnesium doses from supplements commonly cause diarrhoea, nausea and abdominal cramping.1
    Studied for
    Heart failure, heart rhythm and blood pressure (Russian trials)
No data yeta gap in our data, not a negative findingScroll the table for every column

Elemental magnesium in 1,000 mg of each form

mg per 1,000 mg of compound
  • Epsom salt crystals (magnesium sulfate heptahydrate, USP)99mg
  • Epsom salt bath or foot soak99mg
  • Magnesium sulfate in supplement capsules99mg
  • Magnesium sulfate injection (prescription, hospital use)99mg
Arithmetic from the elemental percentages in the forms table. More elemental content per gram says nothing about absorption or tolerance.

What the human research says

Human trials only, one row per outcome. The form line shows what each trial actually tested. Grades follow our evidence-grading policy.

Grade B for sulfate itself. 6 outcomes on this page were tested with sulfate: functional constipation in women (mineral water); treatment response in functional constipation (mineral water); bowel movements and stool consistency in functional constipation; bowel movement frequency in functional constipation; arterial stiffness and blood pressure (daily supplement); gut transit speed and stool weight (oral laxative dose) (RCT, n 244, 2014). Rows that tested other or mixed forms describe magnesium in general, not sulfate. For the overall evidence, see the Magnesium page.

Trial map

7 graded outcomes, largest study first

7 graded outcomes: 4 improved, 1 mixed, 2 no effect. The largest, a 2020 meta-analysis of 307 people on Night-time leg cramps in older adults (magnesium in any form), found no effect.

2. Functional constipation in women (mineral water): improved, 244 people23. Treatment response in functional constipation (mineral water): improved, 226 people36. Bowel movements and stool consistency in functional constipation: improved, 75 people67. Gut transit speed and stool weight (oral laxative dose): improved, 8 people7
5. Bowel movement frequency in functional constipation: mixed, 100 people5
1. Night-time leg cramps in older adults (magnesium in any form): no effect, 307 people14. Arterial stiffness and blood pressure (daily supplement): no effect, 164 people4

1Night-time leg cramps in older adults (magnesium in any form)

No effect · No-effect grade · Meta-analysis, 307 people, 2020

Mostly older adults with idiopathic cramps, presumed nocturnal leg cramps, in 5 trials

Dose, result and source
Every graded outcome on this page, largest study first
OutcomeFoundPeopleStudyWho was studied
Night-time leg cramps in older adults (magnesium in any form)No effect307No-effect grade · Meta-analysis, 2020Mostly older adults with idiopathic cramps, presumed nocturnal leg cramps, in 5 trials
Functional constipation in women (mineral water)Improved244Grade B · RCT, 2014Women aged 18 to 60 with functional constipation (Rome III), recruited by 62 French general practitioners
Treatment response in functional constipation (mineral water)Improved226Grade B · RCT, 2019Outpatients with functional constipation in a multicentre trial
Arterial stiffness and blood pressure (daily supplement)No effect164Grade D · RCT, 2022Overweight or slightly obese adults, mean age 63.2, 63.4% women; four arms (citrate, oxide, sulfate, placebo)
Bowel movement frequency in functional constipationMixed100Grade C · RCT, 2016Adults with functional constipation (Rome III)
Bowel movements and stool consistency in functional constipationImproved75Grade B · RCT, 2017Otherwise healthy adults with functional constipation (106 randomized; 75 in the 500 mL arms)
Gut transit speed and stool weight (oral laxative dose)Improved8Grade D · Crossover RCT, 2000Healthy volunteers (four women, four men), aged about 21
Each circle is one graded outcome on this page, numbered from the largest study down, and placed by how many people it studied (log scale). It shows who was studied and what was found, not advice about your dose.

Every graded row

7 rows

Showing 7 of 7 outcomes. Human trials only.

Grade BFunctional constipation in women (mineral water)Constipation · tested sulfate37.5% vs 21.1% improvedRCT
n 244 · 2014

Improved At week 2, constipation was reduced in 37.5% with 1 L/day of Hépar vs 21.1% with low-mineral water (P = 0.013); 0.5 L/day gave 30.9% (P = 0.099). The 1 L group also had fewer hard stools and used less rescue medication. No serious adverse events.

Design
RCT, n = 244, 2014
Population
Women aged 18 to 60 with functional constipation (Rome III), recruited by 62 French general practitioners
Form
Magnesium sulfate-rich natural mineral water (Hépar) vs low-mineral water
Dose
0.5 L or 1 L/day of Hépar within 1.5 L/day of water
Duration
4 weeks
Source
Reference 6 · PubMed 24342746 (opens PubMed)
Grade BTreatment response in functional constipation (mineral water)Constipation · tested sulfate50% vs 29% respondersRCT
n 226 · 2019

Improved More people responded with Hépar than with low-mineral water at day 14 (50% vs 29%, P = 0.001), and mean time to response was shorter (6.4 vs 7.3 days). Safety results were good.

Design
RCT, n = 226, 2019
Population
Outpatients with functional constipation in a multicentre trial
Form
Magnesium sulfate-rich natural mineral water (Hépar) vs low-mineral water
Dose
1 L/day within 1.5 L/day of water
Duration
14 days
Source
Reference 7 · PubMed 31170680 (opens PubMed)
Grade BBowel movements and stool consistency in functional constipationConstipation · tested sulfateBetter stool consistency and frequencyRCT
n 75 · 2017

Improved Complete spontaneous bowel movements per week tended to rise more with the mineral water (one-sided p = 0.036); spontaneous bowel movements, stool consistency and constipation symptoms improved significantly versus placebo.

Design
RCT, n = 75, 2017
Population
Otherwise healthy adults with functional constipation (106 randomized; 75 in the 500 mL arms)
Form
Natural mineral water rich in magnesium sulfate and sodium sulfate (Donat Mg) vs placebo water
Dose
500 mL/day (300 mL arms stopped at interim analysis)
Duration
6 weeks
Source
Reference 15 · PubMed 26582579 (opens PubMed)
Grade CBowel movement frequency in functional constipationConstipation · tested sulfate+2.02 vs +0.88 a week at 3 weeksRCT
n 100 · 2016

Mixed At the primary 6-week endpoint there was no significant difference (p = 0.163). At 3 weeks, bowel movements rose by 2.02 a week with the mineral water vs 0.88 with tap water (p = 0.005).

Design
RCT, n = 100, 2016
Population
Adults with functional constipation (Rome III)
Form
Carbonated calcium and magnesium sulfate-rich mineral water vs carbonated tap water
Dose
1 L/day (4 × 250 mL)
Duration
6 weeks
Source
Reference 17 · PubMed 27924798 (opens PubMed)
Grade DArterial stiffness and blood pressure (daily supplement)Heart · tested sulfateNo change vs placeboRCT
n 164 · 2022

No effect Magnesium sulfate, like oxide, had no significant effect on carotid-femoral pulse wave velocity or blood pressure. Plasma magnesium rose similarly with all three forms; urinary magnesium rose more with citrate than with oxide or sulfate.

Design
RCT, n = 164, 2022
Population
Overweight or slightly obese adults, mean age 63.2, 63.4% women; four arms (citrate, oxide, sulfate, placebo)
Form
Magnesium sulfate (one of three forms) vs placebo
Dose
450 mg/day magnesium
Duration
24 weeks
Source
Reference 10 · PubMed 35253448 (opens PubMed)
Grade DGut transit speed and stool weight (oral laxative dose)Constipation · tested sulfateFaster transit in all 8Crossover RCT
n 8 · 2000

Improved Magnesium sulfate accelerated duodenum-to-caecum transit and increased faecal weight and fat in every subject; with a meal it also raised peptide YY and inhibited gallbladder emptying.

Design
Crossover RCT, n = 8, 2000
Population
Healthy volunteers (four women, four men), aged about 21
Form
Oral magnesium sulfate vs placebo, fasting and with a meal
Dose
One laxative dose per experiment
Duration
Four experiments
Source
Reference 8 · PubMed 10792122 (opens PubMed)
No effectNight-time leg cramps in older adults (magnesium in any form)Muscle cramps · other or mixed forms−0.18 cramps/week (n.s.)Meta-analysis · 5 studies
n 307 · 2020

No effect Cramps per week did not differ from placebo at 4 weeks (−0.18 cramps/week, 95% CI −0.84 to 0.49; moderate certainty), nor did the share of people with a 25% or better reduction. Evidence in pregnancy-associated cramps was conflicting and at high risk of bias.

Design
Meta-analysis, 5 studies, n = 307, 2020
Population
Mostly older adults with idiopathic cramps, presumed nocturnal leg cramps, in 5 trials
Form
Oral magnesium supplements (forms varied); one trial used intravenous infusions
Dose
As given in each trial
Duration
4 weeks for the main outcomes
Source
Reference 18 · PubMed 32956536 (opens PubMed)

What the numbers add up to

Each point is worked out from the data and the sources numbered on this page, with the arithmetic shown under it. It adds no new data and is not medical advice.

One teaspoon is more than the supplement upper limit

A single 5 g teaspoon provides 495 mg of magnesium, about 1.4 times the 350 mg/day upper limit for supplemental magnesium; the top adult laxative amount (30 g) is about 8.5 times it.15

How we got this: 495 ÷ 350 = 1.41; 30 g × 99 mg/g = 2,970 mg; 2,970 ÷ 350 = 8.5

Why grams of Epsom salt and grams of magnesium differ tenfold

Seven water molecules make up about half the weight of the crystal, so only about a tenth of Epsom salt is magnesium; anhydrous magnesium sulfate is about a fifth.23

How we got this: 7 × 18.0 = 126 g of water per 246.5 g (51%); 24.3 ÷ 246.5 = 9.9% (heptahydrate); 24.3 ÷ 120.4 = 20.2% (anhydrous); PubChem molecular weights

The mineral-water effect in plain numbers

In the 244-woman trial, 16 more women in every 100 improved by week 2 with 1 L a day of the magnesium sulfate-rich water than with low-mineral water.6

How we got this: 37.5% − 21.1% = 16.4 percentage points

Studied doses

Doses used in research, shown against the official limits. They describe the studies, not a recommendation for you.

Studied doses in research, not advice. These are amounts used in research, not personal advice. Your clinician or pharmacist can say what fits you.

Doses studied in human trials

Doses studied in human trials ranged from 0.3 to 1 L a day of magnesium sulfate-rich mineral water for 2 to 6 weeks to 450 mg a day of elemental magnesium as magnesium sulfate for 24 weeks; a transit study used single laxative doses. The Epsom salt label's adult laxative directions are 10 to 30 g a day (990 to 2,970 mg of magnesium) for occasional use. These are research amounts and label directions, not advice.6715171081

When trials measured an effect

3 outcomes
How long trials ran before effects were measured
OutcomeMeasured atBasis
Bowel movement (laxative dose)Generally within 30 minutes to 6 hoursOTC Drug Facts label1
Functional constipation (mineral water)From about week 2; mean time to response 6.4 daysTwo French randomized trials67
Blood magnesium (daily supplement)Measured at 24 weeksFour-arm randomized trial10
When each outcome was assessed in the cited trials. It is not a promise of when anyone will notice a change.

Interactions and safety

Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.

What Magnesium sulfate meets

5 cited rows

Medicines 5

Magnesium sulfate5 medicines · 0 supplements

Supplements and foods 0

No supplement rows in our sources.

  • Drug label or FDA notice
Rows as our evidence profiles cite them; links stay on this site. Line weight shows what backs a row, not how serious an interaction is. No line means none was found in our sources, not that a combination is safe. Not medical advice: ask your pharmacist.

Interactions

5 agents
Published interactions, grouped by kind
AgentWhat the source saysEvidence
Medicines5
Other medicines taken by mouthe.g. Any prescription or OTC medicineThe Epsom salt laxative label says to ask a doctor or pharmacist before use with any other drug and to separate it from other drugs by 2 or more hours, because laxatives may affect how other drugs work.1Drug label
Tetracycline and quinolone antibioticse.g. doxycycline, ciprofloxacinTheir labels say magnesium-containing products impair absorption; ciprofloxacin should be given at least 2 hours before or 6 hours after magnesium antacids.1920Drug label
Oral bisphosphonatese.g. alendronateThe alendronate label says antacids and oral medicines containing multivalent cations such as magnesium interfere with its absorption.21Drug label
Neuromuscular blocking agents (general anaesthesia)e.g. Muscle relaxants used in surgeryThe injection label reports excessive neuromuscular block when parenteral magnesium sulfate is given with a neuromuscular blocking agent.14Drug label
Cardiac glycosidese.g. digoxinThe injection label advises extreme caution in patients on digitalis, because heart block can occur if calcium is needed to treat magnesium toxicity.14Drug label

Check with what you take

Kept on this device only

Add what you take. We list every interaction our evidence profiles cite between Magnesium sulfate and your picks, and between your picks.

Most cited:

Add a supplement or medicine you take to check it against Magnesium sulfate.

Starting from one medicine? The interaction checker rates how serious each pair is, on a curated list of 19 entries for 11 medicines. This cabinet has no severity rating: it lists every interaction row our supplement pages cite.

Not medical advice, and no dosing advice. An empty result means no row we cite names that pair, by name or through a drug class we can link the medicine to, not that a combination is safe. Where our sources name only a group of medicines we cannot match, the cabinet says so. Ask your pharmacist before you combine supplements and medicines.

Who should be careful

6 groups
Kidney disease
The Epsom salt label says to ask a doctor first if you have kidney disease or are on a magnesium-restricted diet; magnesium is cleared only by the kidneys.114
Abdominal pain, nausea or vomiting
The label says to ask a doctor before use with these symptoms, after a change in bowel habits lasting 2 weeks, or after using a laxative for more than a week, and to stop if there is rectal bleeding or no bowel movement.1
Children
Labels give lower amounts for ages 6 to 11 and say children under 6 should not use it without a doctor's advice.14
Bowel conditions and large doses
A 60-year-old woman with a past gastrectomy lost consciousness 30 minutes after swallowing Epsom salts before a colonoscopy, with a blood magnesium of 8.71 mmol/L, despite normal kidney function.13
Pregnancy and breastfeeding
Ask a health professional before using it as a laxative in pregnancy or breastfeeding (label). Prolonged hospital infusions beyond 5 to 7 days have caused low calcium and bone changes in the fetus. LactMed notes that infants absorb oral magnesium poorly, so maternal magnesium is not expected to affect a breastfed baby's blood magnesium.11425
Diabetes and hot soaks
Epsom soak directions say hot or warm soaks should not be used by people with diabetes except on a physician's advice.4
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.

Too much magnesium

3 notes

A laxative dose is many times the supplement upper limit. That is fine for occasional use as directed and dangerous when it is not.

Case reports
A 40-year-old woman with normal kidney function arrived after a cardiac arrest from an accidental Epsom salt overdose, and the case was fatal; another patient survived a blood magnesium of 8.71 mmol/L after swallowing Epsom salts before a colonoscopy.1213
Signs
The injection label lists flushing, sweating, low blood pressure, depressed reflexes, flaccid paralysis, low body temperature, and cardiac and breathing depression as signs of magnesium intoxication; loss of the knee-jerk reflex is an early sign.14
What to do
The OTC label says to get medical help or contact a Poison Control Center right away in case of overdose.1

Hospital uses (prescription)

4 notes

Injected magnesium sulfate is a different product used under monitoring. These results do not apply to Epsom salt.

Pre-eclampsia
In the Magpie trial (10,141 women in 33 countries), magnesium sulfate cut eclampsia by 58% versus placebo; a Cochrane review of six trials (11,444 women) found it more than halved eclampsia, with flushing in 24% vs 5%.1622
Acute asthma
A single 1.2 g or 2 g infusion reduced hospital admissions in adults whose asthma attack had not responded enough to first-line treatment (14 trials, 2,313 people); in children the evidence is limited to small trials.2324
Fetal bone effects
The injection label warns that continuous use beyond 5 to 7 days in pregnancy can cause low calcium and bone abnormalities in the developing baby, including fractures.14
Breastfeeding
NIH's LactMed says intravenous magnesium raises milk magnesium only slightly and is not expected to affect the infant's blood magnesium, though 24-hour postpartum infusions may delay the start of lactation.25

How to read the label

What to check on the Supplement Facts panel.

How to read the label

4 checks
Grams to magnesium

Epsom salt is about 9.9% magnesium. The label converts it for you: each 5 g teaspoon contains 495 mg of magnesium.12

Laxative use is occasional use

Adult directions of 10 to 30 g a day are far above the 350 mg/day supplement upper limit, and the label says to ask a doctor after a week of laxative use.15

Soak or swallow

The same pack may carry laxative directions and soaking directions. The constipation evidence is for swallowing it (or mineral water), not for baths.1411

USP grade

OTC Epsom salt is labelled magnesium sulfate USP with no inactive ingredients.1

Frequently asked questions

Short answers to common questions; the numbers link to the sources below.

Common questions

8 answers
Do Epsom salt baths raise magnesium levels?

There is no controlled evidence that they do. A 2017 review concluded that claims for transdermal magnesium, including salt baths, are scientifically unsupported. A warm bath may feel good, but it is not a proven way to raise body magnesium.11

Does Epsom salt work for constipation?

It is an approved OTC saline laxative for occasional constipation and generally works in 30 minutes to 6 hours. Mineral waters rich in magnesium sulfate also helped people with functional constipation in several trials (grade B).16715

Do Epsom salt soaks help sore muscles or cramps?

We found no controlled trial of Epsom soaks for sore muscles. Labels list soaks for minor sprains and bruises. For night-time leg cramps, magnesium supplements did no better than placebo in a Cochrane review.41811

How much Epsom salt is safe to drink?

Label directions vary by brand; one adult label gives 10 to 30 g a day dissolved in 8 oz of water, for occasional use, and says to ask a doctor first with kidney disease or a magnesium-restricted diet. Swallowing much more has caused dangerous and even fatal magnesium levels.11213

Can Epsom salt cause magnesium poisoning?

Yes, if too much is swallowed. Case reports describe a fatal and a near-fatal case in people with normal kidneys. Signs include low blood pressure, weak reflexes, drowsiness and slowed breathing; get emergency help.121314

Is magnesium sulfate a good daily magnesium supplement?

It can raise blood magnesium: 450 mg/day as sulfate raised plasma magnesium about as much as citrate or oxide in a 24-week trial, though urinary magnesium rose less than with citrate. Most supplements use other forms.10

Does Epsom salt interact with medicines?

The label says to keep it 2 or more hours away from other drugs and to ask a pharmacist first. Magnesium also reduces absorption of some antibiotics (doxycycline, ciprofloxacin) and of alendronate.1192021

Can I use Epsom salt while pregnant or breastfeeding?

The laxative label says to ask a health professional before use in pregnancy or breastfeeding. The pregnancy warnings for prolonged hospital infusions (more than 5 to 7 days) do not describe occasional oral laxative use. LactMed notes that infants absorb oral magnesium poorly, so maternal magnesium is not expected to raise a breastfed baby's magnesium.11425

Sources

25 sources: trial abstracts, NIH fact sheets and reference databases. Numbers in the text link here.

Sources cited on this page

25 sources
  1. 1Epsom Salt (magnesium sulfate USP, heptahydrate) granule: OTC Drug Facts label (saline laxative)Label Vi-Jon, LLC via DailyMed (NLM), 2026 · dailymed.nlm.nih.gov
  2. 2PubChem compound summary: Magnesium sulfate heptahydrate (CID 24843)Database National Library of Medicine, 2026 · pubchem.ncbi.nlm.nih.gov
  3. 3PubChem compound summary: Magnesium sulfate (CID 24083)Database National Library of Medicine, 2026 · pubchem.ncbi.nlm.nih.gov
  4. 4Epsom Salt (magnesium sulfate) granule, for solution: OTC label with soaking directionsLabel Target Corporation via DailyMed (NLM), 2026 · dailymed.nlm.nih.gov
  5. 5Perspective: Call for Re-evaluation of the Tolerable Upper Intake Level for Magnesium Supplementation in Adults.Review Adv Nutr, 2023 · PMID 37487817
  6. 6Efficacy and safety of a magnesium sulfate-rich natural mineral water for patients with functional constipation.RCT Clin Gastroenterol Hepatol, 2014 · PMID 24342746
  7. 7Time to treatment response of a magnesium- and sulphate-rich natural mineral water in functional constipation.RCT Nutrition, 2019 · PMID 31170680
  8. 8The osmotic laxative magnesium sulphate activates the ileal brake.Crossover RCT Aliment Pharmacol Ther, 2000 · PMID 10792122
  9. 9Magnesium Sulfate-Rich Natural Mineral Waters in the Treatment of Functional Constipation-A Review.Review Nutrients, 2020 · PMID 32664341
  10. 10Effects of Magnesium Citrate, Magnesium Oxide, and Magnesium Sulfate Supplementation on Arterial Stiffness: A Randomized, Double-Blind, Placebo-Controlled Intervention Trial.RCT J Am Heart Assoc, 2022 · PMID 35253448
  11. 11Myth or Reality-Transdermal Magnesium?Review Nutrients, 2017 · PMID 28788060
  12. 12Fatal Hypermagnesemia: an acute ingestion of Epsom Salt in a patient with normal renal function.Observational Caspian J Intern Med, 2018 · PMID 30510660
  13. 13Case report: Near-fatal hypermagnesemia resulting from the use of Epsom salts in a patient with normal renal function.Observational Front Med (Lausanne), 2024 · PMID 39021819
  14. 14Magnesium Sulfate Injection, USP 50%: prescribing informationLabel Fresenius Kabi USA via DailyMed (NLM), 2026 · dailymed.nlm.nih.gov
  15. 15Efficacy and safety of a natural mineral water rich in magnesium and sulphate for bowel function: a double-blind, randomized, placebo-controlled study.RCT Eur J Nutr, 2017 · PMID 26582579
  16. 16Do women with pre-eclampsia, and their babies, benefit from magnesium sulphate? The Magpie Trial: a randomised placebo-controlled trial.RCT Lancet, 2002 · PMID 12057549
  17. 17Effects of Sulfate-Rich Mineral Water on Functional Constipation: A Double-Blind, Randomized, Placebo-Controlled Study.RCT Forsch Komplementmed, 2016 · PMID 27924798
  18. 18Magnesium for skeletal muscle cramps.Meta-analysis Cochrane Database Syst Rev, 2020 · PMID 32956536
  19. 19Ciprofloxacin tablets: prescribing information (administration with multivalent cations)Label DailyMed (NLM), 2026 · dailymed.nlm.nih.gov
  20. 20Doxycycline hyclate capsules: prescribing information (drug interactions)Label DailyMed (NLM), 2026 · dailymed.nlm.nih.gov
  21. 21Alendronate sodium tablets: prescribing information (calcium supplements and antacids)Label DailyMed (NLM), 2026 · dailymed.nlm.nih.gov
  22. 22Magnesium sulphate and other anticonvulsants for women with pre-eclampsia.Meta-analysis Cochrane Database Syst Rev, 2010 · PMID 21069663
  23. 23Intravenous magnesium sulfate for treating adults with acute asthma in the emergency department.Meta-analysis Cochrane Database Syst Rev, 2014 · PMID 24865567
  24. 24Intravenous magnesium sulfate for treating children with acute asthma in the emergency department.Meta-analysis Cochrane Database Syst Rev, 2016 · PMID 27126744
  25. 25Magnesium Sulfate. Drugs and Lactation Database (LactMed).Database National Institute of Child Health and Human Development, 2026 · PMID 30000398

What changed

Every dated change to this page: what changed, who made it and why.

Change log

2 entries
Dated changes to this page
DateChangeWho and why
definition, safety, FAQA–Z audit. PubMed search for 2023–2026 trials and meta-analyses of oral magnesium sulfate or sulfate-rich mineral water found none to add. Shortened the definition to 140 characters; added breastfeeding to the pregnancy note and FAQ from the LactMed magnesium sulfate record (already source 25); cut the FAQ from 11 to 8 by removing three items already covered in the definition, evidence and hospital-use sections.Writer: Kymata Health EditorialTrigger: A–Z page audit
allNew page. Every PMID checked with NCBI E-utilities (esummary title, year and journal) and its abstract read with efetch, including the LactMed record. OTC Epsom salt labels, the magnesium sulfate injection label and the ciprofloxacin, doxycycline and alendronate labels read on DailyMed on the date shown; molecular weights from PubChem. Hospital injection uses are described in their own section and not graded as supplement evidence.Writer: Kymata Health EditorialTrigger: new page

Supplement & Nutrient Encyclopedia. Educational information only; it is not medical advice and does not replace a clinician. Dietary supplements are not intended to diagnose, treat, cure or prevent any disease. Statements about supplements have not been evaluated by the FDA.

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