Omega-3 (fish oil, krill oil, algal oil)
Summary
Fish, krill and algal oils supply EPA and DHA, which lowered triglycerides about 15% in a Cochrane review (high certainty). At about 1 g/day, large trials found no drop in major heart events, and pooled trials link supplements to more irregular heart rhythm, especially above 1 g/day. Krill and fish oil raise blood EPA+DHA similarly.
Key facts
- Fish oil (natural triglyceride, ethyl ester, re-esterified triglyceride), krill oil (mainly phospholipids), algal oil (mainly DHA, triglyceride form)[1]
- Triglycerides ↓ about 15% (high-certainty Cochrane evidence); heart and blood-vessel events ↔ (no effect); irregular heart rhythm risk ↑ (harm, A); eye comfort in dry eye ↔ (no effect)[2][3][4][5]
- About 0.5 to more than 5 g/day EPA+DHA in outcome trials; most supplement trials used about 1 g/day[2][3]
- 1,000 mg fish oil usually provides about 180 mg EPA + 120 mg DHA; algal oil 100–300 mg DHA per serving[1]
- No RDA or UL for EPA/DHA. Adult AI applies to ALA only: 1.6 g (men), 1.1 g (women). FDA: supplements providing no more than 5 g/day EPA+DHA are safe when used as recommended[1]
- 6 graded human outcomes on this page
- October 9, 2026
How it works
EstablishedEPA and DHA lower blood triglycerides in a dose-dependent way (established in trials). At high intakes they reduce platelet aggregation, which can lengthen bleeding time (established). Effects on heart rhythm, inflammation and the eye are proposed and inconsistent in trials.[2][1]
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 |
|---|---|---|---|---|
| Heart | Grade A | Worsened | Meta-analysis · n = 81,210 · 2021 | 5 rows |
| Healthy aging | No effect | No effect | RCT · n = 535 · 2018 | 1 row |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Irregular heart rhythm risk | Worsened | Grade A | Meta-analysis · n = 81,210 · 2021 7 cardiovascular-outcome RCTs (≥500 participants, ≥1 year), mean age 65 Form: marine omega-3 fatty acids (formulation varied by trial) · Dose: ≤1 g/day in trials with 72.6% of participants; >1 g/day in the rest · Duration: Weighted mean 4.9 years Omega-3 was associated with more atrial fibrillation (HR 1.25). Risk was higher in trials using >1 g/day (HR 1.49) than ≤1 g/day (HR 1.12); each extra 1 g/day raised the HR by about 11%. | 34612056 (opens PubMed)Source [4] |
| Blood EPA+DHA: krill oil vs fish oil at matched amounts | No effect | Grade C | RCT · n = 66 · 2015 Healthy adults, 22 per arm Form: krill oil vs fish oil ethyl ester vs fish oil triglyceride · Dose: 1.3 g/day EPA+DHA (about 816 mg EPA + 522 mg DHA) in every arm · Duration: 4 weeks No significant difference in plasma EPA+DHA at week 4 (p = 0.052) or red-cell EPA+DHA; bioavailability differed by less than 24% between products. | 26328782 (opens PubMed)Source [6] |
| Blood DHA: algal oil capsules vs cooked salmon | No effect | Grade D | RCT · n = 32 · 2008 Healthy men and women aged 20–65 Form: algal-oil DHA capsules vs assayed cooked salmon · Dose: 600 mg/day DHA · Duration: 2 weeks DHA rose about 80% in plasma phospholipids and about 25% in red cells in both groups; the two sources were bioequivalent. | 18589030 (opens PubMed)Source [8] |
| Heart and blood-vessel events, pooled trials (all doses) | No effect | No effect | Meta-analysis · n = 140,482 · 2020 86 RCTs (162,796 participants) of adults at varying cardiovascular risk; cardiovascular-event analysis in 43 RCTs Form: long-chain omega-3 (EPA/DHA), mostly capsules · Dose: 0.5 to more than 5 g/day (19 trials gave at least 3 g/day) · Duration: 12 to 88 months Little or no effect on cardiovascular events (RR 0.96), all-cause mortality (RR 0.97) or stroke (high/moderate certainty). Possible slight reduction in coronary heart disease events and mortality (low/moderate certainty). Triglycerides fell by about 15%, dose-dependently (high certainty). | 32114706 (opens PubMed)Source [2] |
| Heart and blood-vessel events at 1 g/day (VITAL) | No effect | No effect | RCT · n = 25,871 · 2019 US men 50+ and women 55+ at usual risk (VITAL) Form: marine omega-3 (fish oil) · Dose: 1 g/day marine n-3 fatty acids · Duration: Median 5.3 years Major cardiovascular events HR 0.92 (95% CI 0.80–1.06) and invasive cancer HR 1.03; neither significant. Secondary: total myocardial infarction HR 0.72 (0.59–0.90). No excess bleeding. | 30415637 (opens PubMed)Source [3] |
| Eye comfort and tear-film measures | No effect | No effect | RCT · n = 535 · 2018 Adults with moderate-to-severe dry eye disease (DREAM) Form: fish-derived EPA + DHA vs olive oil placebo · Dose: 3,000 mg/day EPA + DHA · Duration: 12 months Symptom score improved similarly in both groups (-13.9 vs -12.5 points; difference not significant). No differences in staining, tear break-up time or Schirmer's test. | 29652551 (opens PubMed)Source [5] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Fish oil (triglyceride, ethyl ester, re-esterified triglyceride) | Varies[1] | Natural and re-esterified triglycerides and free fatty acids have somewhat higher bioavailability than ethyl esters, but all forms raise plasma EPA and DHA.[1][6] | Usually mild: fishy taste, bad breath, heartburn, nausea, GI discomfort, diarrhea, headache, odoriferous sweat.[1] | Cardiovascular events, triglycerides, atrial fibrillation, dry eye |
| Krill oil (phospholipid form) | Varies[1] | Matched-dose trials show similar blood EPA+DHA to fish oil; some small studies suggest somewhat higher uptake, others do not.[6][7][1] | No head-to-head tolerance data in our sources. Krill are crustaceans; prescription omega-3 labels advise caution with fish or shellfish hypersensitivity.[9] | Blood EPA+DHA levels (bioavailability) |
| Algal oil (mainly DHA) | Varies[1] | DHA from algal-oil capsules matched cooked salmon in a small 2-week trial.[8][1] | Generally well tolerated in the comparison trial.[8] | Blood DHA levels; the vegan option |
Studied doses, daily needs and limits
Doses studied in human trials ranged from about 0.5 to more than 5 g/day of EPA plus DHA, for 2 weeks to about 7 years. Most large supplement trials tested about 1 g/day; 3–4 g/day appeared in dry eye and prescription-strength trials.[2][3][5][8]
Intake reference: No RDA, AI or UL for EPA or DHA. The adult AI covers ALA only: 1.6 g/day for men and 1.1 g/day for women (1.4 g in pregnancy, 1.3 g in breastfeeding).[1] No UL. EFSA considers up to about 5 g/day EPA+DHA long-term safe, and FDA has concluded supplements providing no more than 5 g/day are safe when used as recommended. NIH ODS fact sheet (updated August 22, 2025).
Recommended intake and upper limit by age and sex
| Age | Male | Female | Pregnancy | Lactation | Upper limit (UL) |
|---|---|---|---|---|---|
| 0–6 months (AI) | 0.5 g | 0.5 g | — | — | Not set |
| 7–12 months (AI) | 0.5 g | 0.5 g | — | — | Not set |
| 1–3 years (AI) | 0.7 g | 0.7 g | — | — | Not set |
| 4–8 years (AI) | 0.9 g | 0.9 g | — | — | Not set |
| 9–13 years (AI) | 1.2 g | 1 g | — | — | Not set |
| 14–18 years (AI) | 1.6 g | 1.1 g | 1.4 g | 1.3 g | Not set |
| 19–50 years (AI) | 1.6 g | 1.1 g | 1.4 g | 1.3 g | Not set |
| 51+ years (AI) | 1.6 g | 1.1 g | — | — | Not set |
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
- Warfarin and similar anticoagulantsNIH fact sheetHigh-dose fish oil has antiplatelet effects and might raise INR with warfarin, although most research finds 3–6 g/day does not significantly change anticoagulation. Prescription omega-3 labels advise periodic monitoring.[1]
- Anticoagulants or antiplatelet drugsDrug labelOmega-3 acids may prolong bleeding time; the prescription label advises periodic monitoring when combined.[9]
Who should be careful
- Atrial fibrillation or high cardiovascular risk
- Pooled outcome trials link omega-3 supplements to more atrial fibrillation, especially above 1 g/day, and the prescription label notes more AF recurrences in people with existing AF.[4][1][9]
- Bleeding risk and surgery
- Doses of 2–15 g/day EPA and/or DHA might lengthen bleeding time. Tell your surgeon or dentist, and your clinician if you take blood thinners.[1]
- Fish or shellfish allergy
- Fish oil and krill oil come from fish and crustaceans; prescription omega-3 labels advise caution with known fish or shellfish hypersensitivity. Algal oil is plant-derived.[9][1]
- Pregnancy and breastfeeding
- DHA matters in pregnancy. A Cochrane review cited by ODS found supplements slightly lengthened gestation and raised birth weight, with no consistent effects on other outcomes.[1]
- Immune function at high doses
- The IOM noted 900 mg EPA + 600 mg DHA per day or more for several weeks might reduce immune function.[1]
- Before you start
- Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
Cost per studied dose, without the sales pitch
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Cost per studied dose
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Articles about Omega-3 (fish oil, krill oil, algal oil)
Plain-English answers to common questions, built on the evidence above.
Frequently asked questions
Is krill oil better than fish oil?
Not clearly. When EPA and DHA amounts were matched, a 4-week trial of 66 adults found similar blood levels with krill oil and two fish oil forms. Small single-dose studies suggest krill may be absorbed slightly better, but results are inconsistent. Krill capsules usually contain less EPA+DHA per serving.[6][7][1]
Is algae oil as good as fish oil for DHA?
For raising DHA, a small trial found 600 mg/day DHA from algal-oil capsules matched cooked salmon. Algal oils mostly provide DHA (100–300 mg per serving), sometimes with some EPA.[8][1]
How much EPA and DHA is in a fish oil capsule?
A typical 1,000 mg fish oil capsule provides about 180 mg EPA and 120 mg DHA, roughly 300 mg combined. Products vary widely, so compare the EPA and DHA lines on the label, not the total oil weight.[1]
Does fish oil lower heart attack and stroke risk?
At about 1 g/day, large trials such as VITAL found no significant reduction in major cardiovascular events, and a Cochrane review of 86 trials found little or no effect on cardiovascular events or stroke, with a possible slight reduction in coronary heart disease events.[3][2]
Can fish oil cause atrial fibrillation?
A 2021 meta-analysis of 7 large trials (81,210 people) found a 25% higher risk of atrial fibrillation with omega-3 supplements, rising to 49% in trials using more than 1 g/day.[4]
Do fish oil supplements contain mercury?
ODS reports omega-3 supplements have not been found to contain methylmercury because it is removed during processing and purification.[1]
Sources
- [1]Omega-3 Fatty Acids: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2025. fact-sheet
- [2]Omega-3 fatty acids for the primary and secondary prevention of cardiovascular disease.. Cochrane Database Syst Rev, 2020. meta-analysis
- [3]Marine n-3 Fatty Acids and Prevention of Cardiovascular Disease and Cancer.. N Engl J Med, 2019. RCT
- [4]Effect of Long-Term Marine ɷ-3 Fatty Acids Supplementation on the Risk of Atrial Fibrillation in Randomized Controlled Trials of Cardiovascular Outcomes: A Systematic Review and Meta-Analysis.. Circulation, 2021. meta-analysis
- [5]n-3 Fatty Acid Supplementation for the Treatment of Dry Eye Disease (DREAM). N Engl J Med, 2018. RCT
- [6]Similar eicosapentaenoic acid and docosahexaenoic acid plasma levels achieved with fish oil or krill oil in a randomized double-blind four-week bioavailability study.. Lipids Health Dis, 2015. RCT
- [7]Incorporation of EPA and DHA into plasma phospholipids in response to different omega-3 fatty acid formulations--a comparative bioavailability study of fish oil vs. krill oil.. Lipids Health Dis, 2011. crossover-RCT
- [8]Algal-oil capsules and cooked salmon: nutritionally equivalent sources of docosahexaenoic acid.. J Am Diet Assoc, 2008. RCT
- [9]LOVAZA (omega-3-acid ethyl esters) capsule: prescribing information (Warnings 5.2, 5.3; Drug Interactions 7.1). DailyMed, U.S. National Library of Medicine, 2026. label
What changed
- First draft. Every evidence-row PMID checked with NCBI E-utilities (esummary + efetch abstract) and re-read in an independent adversarial pass.
- Added intake table from the NIH ODS fact sheet (checked Oct 9 2026), cost-per-studied-dose method, labeled analysis derived from cited rows.