Horsetail
Summary
Horsetail is a silica- and potassium-rich herb that EU regulators accept, on long-standing use, for flushing the urinary tract and for superficial wounds. Small Brazilian trials found a diuretic effect similar to hydrochlorothiazide and blood-pressure falls without a placebo arm. The raw plant contains thiaminase; heart or kidney disease and lithium call for caution.
Key facts
- The EMA's 2016 EU herbal monograph accepts horsetail, on long-standing use, to increase urine flow and flush the urinary tract in minor urinary complaints and as supportive care for superficial wounds. It is not recommended under age 12 or where reduced fluid intake is advised.[1]
- In 36 healthy men, 900 mg/day of dry extract for 4 days increased urine output more than placebo and about as much as hydrochlorothiazide 25 mg, without significant changes in electrolyte excretion.[3]
- Fresh and dried horsetail contains thiaminase, an enzyme that breaks down vitamin B1. Industrial aqueous and ethanol extracts showed none, and Health Canada requires products to be thiaminase-free. Toxicity of unheated raw powder cannot be excluded.[2]
- Marsh horsetail (Equisetum palustre) contains the toxic alkaloid palustrine; the European Pharmacopoeia includes a test to exclude it, and the EMA advises using quality-controlled horsetail only.[2]
- The herb contains 5–7.7% silicic acid, about a tenth of it water-soluble; in one experiment tea made without boiling extracted at most 0.3% of the plant weight as silicon.[2]
- 3 graded human outcomes on this page
- October 13, 2026
How it works
ProposedHow horsetail increases urine flow is not established. The EMA assessment suggests its flavonoids and high potassium content may contribute. In an uncontrolled 7-day study in 10 volunteers, an aqueous extract raised urinary Tamm-Horsfall protein (uromodulin) by up to 300% and urine electrolytes indicated a diuretic effect. Authors of lab studies suggested that silica and saponins may explain wound contraction (preclinical).[2][6]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
At a glance: strongest evidence per outcome
| Outcome | Grade | Effect | Strongest evidence | Rows |
|---|---|---|---|---|
| Wounds | Grade D | Improved | RCT · n = 108 · 2015 | 1 row |
| Blood pressure | Grade D | Improved | RCT · n = 58 · 2022 | 1 row |
| Diuretic effect | Grade D | Improved | Crossover RCT · n = 36 · 2014 | 1 row |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Episiotomy wound recovery and pain (3% ointment) | Improved | Grade D | RCT · n = 108 · 2015 First-time mothers after episiotomy (Iran); 54 per group Form: 3% Equisetum arvense ointment (topical) · Dose: Applied to the episiotomy wound (frequency not stated in the abstract) · Duration: 10 days Wound scores (REEDA: redness, swelling, bruising, discharge, edge closure) and pain were lower than with placebo ointment at days 5 and 10; the adjusted pain difference at day 5 was −2.3 points. | 26019907 (opens PubMed)Source [5] |
| Blood pressure in stage 1 hypertension | Improved | Grade D | RCT · n = 58 · 2022 Adults aged 25–65 with stage 1 hypertension (Brazil) Form: Standardised dry extract of Equisetum arvense · Dose: 900 mg/day vs hydrochlorothiazide 25 mg/day · Duration: 3 months Systolic and diastolic pressure fell by an average of 12.6 and 8.1 mmHg with horsetail, with no significant difference from hydrochlorothiazide and no significant changes in kidney, liver or blood tests. | 35168030 (opens PubMed)Source [4] |
| Urine output over 24 hours (diuretic effect) | Improved | Grade D | Crossover RCT · n = 36 · 2014 Healthy men (Brazil) Form: Standardised dry extract of Equisetum arvense · Dose: 900 mg/day · Duration: 4 days per period, with 10-day washouts Urine output rose more than with placebo (corn starch) and about as much as with hydrochlorothiazide 25 mg/day, without significant changes in electrolyte excretion; adverse events were rare and minor, and laboratory tests did not change. | 24723963 (opens PubMed)Source [3] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Aqueous dry extract (capsules or tablets) | Varies[1][3][2] | No human pharmacokinetic data were found. The EMA monograph lists a 4–7:1 aqueous dry extract at 1,080–1,110 mg/day for traditional use; trials used 900 mg/day of a standardised extract. Industrial aqueous extracts showed no thiaminase activity.[1][3][2] | The EMA lists mild gastrointestinal complaints and allergic reactions (rash, facial swelling) at unknown frequency. Adverse events in trials were rare and minor.[1][3][4] | Urine output in healthy men and stage 1 hypertension (no placebo) |
| Tea (infusion or decoction) | Varies[1][2] | The EMA monograph lists 1–4 g of herb in 150 ml of boiling water, 3–4 times a day (3–12 g a day). Heating inactivates thiaminase (50% at about 70°C, fully at 80°C).[1][2] | As for other forms: mild gastrointestinal complaints and allergic reactions are listed at unknown frequency.[1] | Traditional use for urinary flushing (no trials of tea) |
| Raw powdered herb | Varies[2] | Unheated, unextracted plant powder can keep its thiaminase activity.[2] | The EMA says toxicity of raw powder cannot be excluded, although it is not expected at traditional amounts.[2] | No trials |
| Ointment, compress or wound rinse | Varies[5][1] | Applied to the skin; absorption was not measured. The EMA monograph lists a decoction of 10 g herb in 1 litre of water for dressings and rinses.[5][1] | The episiotomy trial abstract does not report adverse event results; allergic reactions are listed by the EMA at unknown frequency.[5][1] | Episiotomy wound recovery and pain; traditional supportive care for superficial wounds |
Studied doses
Doses studied in human trials ranged from 900 mg/day of a standardised dry extract for 4 days (healthy men) to the same dose for 3 months (stage 1 hypertension). The EMA's traditional-use monograph lists 1,080–1,110 mg/day of a 4–7:1 aqueous dry extract, or 3–12 g/day of herb as tea, over 2–4 weeks.[3][4][1]
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines
- LithiumCase reportA woman on lithium developed lithium toxicity (4.5 mmol/L) after 2–3 weeks of a herbal blend containing several diuretic herbs, including a related horsetail species (Equisetum hyemale). The EMA said no conclusion can be drawn for field horsetail from this case, but notes that people on lithium who use diuretic herbs may become dehydrated and toxic.[2]
- Drugs cleared by CYP2C8 or CYP1A2ReviewAqueous extracts inhibited CYP2C8 and CYP1A2 in lab studies, which the EMA considered potentially clinically relevant (trimethoprim was named as an example). No documented human cases, so no warning was added to the monograph.[2]
Medicines that can lower levels
- Thiamine (vitamin B1)ReviewThiaminase in fresh and dried plant breaks down thiamine; heat-processed preparations and aqueous or ethanol extracts showed no activity.[2]
Who should be careful
- Severe heart or kidney disease
- The EMA monograph contraindicates horsetail where reduced fluid intake is recommended, such as severe heart or kidney disease.[1]
- Impaired potassium handling by the kidneys
- The EMA assessment cites advice that people with known impaired renal potassium handling be cautious with potassium-rich herbs such as horsetail because of the risk of severe hyperkalaemia.[2]
- Urinary warning signs
- The EMA monograph says fever, painful urination, spasms or blood in the urine need assessment by a doctor or qualified health practitioner, as do symptoms lasting more than 1 week. With preparations other than tea, it advises drinking enough fluid.[1]
- People on lithium
- Diuretic herbs may cause dehydration and lithium toxicity (one case report with a multi-herb blend that included a related species).[2]
- Pregnancy and breastfeeding
- Safety has not been established, and the EMA does not recommend use. Topical products should not be applied to the breast while breastfeeding.[1]
- Children under 12
- Not recommended in the EMA monograph.[1]
- Thiamine deficiency
- Health Canada's thiaminase-free requirement reflects concern that thiaminase could deplete vitamin B1 in people who are already deficient.[2]
- Low blood sodium
- A 2021 case report described low blood sodium, caused by poor intake and SIADH, that was possibly made worse by horsetail.[7]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
Frequently asked questions
Does horsetail work as a diuretic?
In one small trial, 900 mg/day of dry extract for 4 days increased urine output in 36 healthy men more than placebo and about as much as hydrochlorothiazide 25 mg. EU regulators accept it for flushing the urinary tract on the basis of traditional use.[3][1]
Can horsetail lower blood pressure?
One 3-month trial in 58 people with stage 1 hypertension found falls of 12.6/8.1 mmHg, similar to hydrochlorothiazide, but it had no placebo group. That is not enough to rely on, and high blood pressure needs clinician-managed care.[4]
Is horsetail a good source of silica?
The herb contains 5–7.7% silicic acid, but only about a tenth is water-soluble, and tea made without boiling extracted at most 0.3% of the plant as silicon in one experiment. See the silica page for the evidence on silicon supplements.[2]
What is thiaminase, and does it matter?
Thiaminase is an enzyme in fresh and dried horsetail that breaks down vitamin B1. Heat and extraction remove it, Health Canada requires thiaminase-free products, and the EMA says raw powder toxicity cannot be excluded.[2]
Is horsetail safe in pregnancy?
Safety has not been established, and the EMA does not recommend horsetail during pregnancy or breastfeeding.[1]
Sources
- [1]European Union herbal monograph on Equisetum arvense L., herba (EMA/HMPC/278091/2015). European Medicines Agency (HMPC), 2016. guideline
- [2]Assessment report on Equisetum arvense L., herba (EMA/HMPC/278089/2015). European Medicines Agency (HMPC), 2016. guideline
- [3]Randomized, Double-Blind Clinical Trial to Assess the Acute Diuretic Effect of Equisetum arvense (Field Horsetail) in Healthy Volunteers.. Evid Based Complement Alternat Med, 2014. crossover-RCT
- [4]Antihypertensive effect of Equisetum arvense L.: a double-blind, randomized efficacy and safety clinical trial.. Phytomedicine, 2022. RCT
- [5]The effect of equisetum arvense (horse tail) ointment on wound healing and pain intensity after episiotomy: a randomized placebo-controlled trial.. Iran Red Crescent Med J, 2015. RCT
- [6]Aqueous extract from Equisetum arvense stimulates the secretion of Tamm-Horsfall protein in human urine after oral intake.. Phytomedicine, 2022. open-label
- [7]Hyponatremia Secondary to Decreased Oral Intake and SIADH and Possibly Exacerbated by Horsetail (Equisetum arvense).. Can J Hosp Pharm, 2021. observational
What changed
- Initial editorial evidence review: every PMID checked against NCBI esummary (title, year, no retraction or expression-of-concern flag); doses, sample sizes and durations re-read against the PubMed abstract or open full text; grades kept conservative; interactions limited to what a cited source supports.
- Adversarial review: clarified that the lithium case involved a multi-herb diuretic blend (EMA: no conclusion possible for field horsetail) and added the EMA's urinary warning signs and fluid-intake advice.