Supplements › Fatty acid or lipid

Fish oil

An oil from fatty fish, rich in the omega-3 fats EPA and DHA. Most studied for triglycerides, which it lowers by about 15%, and for heart disease.

Strongest evidenceHeart healthStrong, 337 studies
Median dose in studies3,000 mg a dayFrom 558 study groups
A typical 1000 mg capsuleAbout 300 mg of omega-3180 mg of EPA and 120 mg of DHA. Products vary widely.
Upper limitNone setEFSA and the US FDA consider up to 5 g of EPA and DHA a day safe for adults.
Ask a doctor or pharmacist first

Ask a doctor or pharmacist first if you:

  • take warfarin or a similar blood thinner: fish oil may prolong clotting times, and prescription omega-3 labels call for checks of INR, a clotting test[1]
  • have heart disease or a high risk of it: more than 1.5 g of EPA and DHA a day raised atrial fibrillation, an irregular heartbeat, in such people[2]
  • use cod liver oil, or are pregnant or breastfeeding: cod liver oil also contains vitamins A and D, which can be harmful in excess[3]
  • take prescribed medicine for rheumatoid arthritis, or may have depression: fish oil should not replace care from a health care provider[3]

Overview

What is fish oil?

Fish oil is a dietary supplement made from the fat of fish. It contains EPA and DHA, two long-chain omega-3 fats.[1] Fish do not make these fats themselves: microalgae make them, and fish collect them through the food chain.[1]

A typical 1000 mg capsule holds about 180 mg of EPA and 120 mg of DHA.[1] Related products are cod liver oil, which also has vitamins A and D, krill oil and algal oil, the vegetarian source.[3] Cold-water fatty fish such as salmon, mackerel, herring and sardines are rich food sources.[1]

What is fish oil used for?

Fish oil is used for heart health, high triglycerides and other conditions.[4] The evidence is clearest for triglycerides, and many other uses have weak or negative results.[5][1]

What the best reviews found:

  • Triglycerides: about 15% lower, with high-certainty evidence, in a Cochrane review.[5]
  • Heart disease: 9% fewer coronary events in the same review, with low-certainty evidence, and little or no effect on all cardiovascular events, stroke or death from any cause.[5]
  • Blood pressure: 2.6 mmHg lower in the top number at 2 to 3 g a day in one review.[6] The Cochrane review found little or no effect.[5]
  • Pregnancy: 11% fewer preterm births in a Cochrane review.[7] A later trial of 5,486 pregnancies saw no fall in early preterm birth.[8]
  • Joints: a small fall in pain from osteoarthritis, and a limited effect in rheumatoid arthritis.[9][10]
  • Depression: a small effect, unlikely to matter in daily life, with very low-certainty evidence.[11]
  • Fatty liver: better GGT, a liver enzyme, and possibly less fat on ultrasound. There was no significant change in two other liver enzymes (AST and ALT) or in fat measured by MRI.[12]

Trials do not support fish oil for preventing memory decline, for slowing macular degeneration, for ADHD in children or for losing weight.[13][1][14][15] Results for dry eyes are mixed, and a 12-month trial of 535 people found no benefit.[1][16]

Is fish oil safe?

For most adults, yes, at the doses studied. EFSA found no safety concern for adults at up to 5 g of EPA and DHA a day.[17] LiverTox rates omega-3 fats as an unlikely cause of liver injury.[4]

Side effects are usually mild: unpleasant taste, bad breath, heartburn, nausea, stomach discomfort, diarrhea, headache and smelly sweat.[1]

Two risks grow with the dose. In people at high heart risk, more than 1.5 g a day raised the chance of atrial fibrillation, an irregular heartbeat.[2] High doses of purified EPA raised bleeding by 0.6 percentage points, while omega-3 fats overall did not raise it.[18]

People who take a blood thinner, have a heart condition, are pregnant or use cod liver oil should ask a doctor or pharmacist first.[1][3]

How it works

Fish oil gives the body EPA and DHA, two long-chain omega-3 fats.[1] The body makes very little of them itself: it converts less than 15% of ALA, the omega-3 fat in plants.[1]

  1. Gut absorption

    Enzymes split the oil in the gut, and bile salts carry the pieces to the cells of the gut wall. About 95% is absorbed, similar to other fats.[1]

  2. Transport

    Gut cells pack the fats into chylomicrons, particles that pass through the lymph into the blood. Lipoprotein particles then carry them to the organs.[1]

  3. Cell membranes

    EPA and DHA become part of cell membranes. DHA is especially high in the retina, the brain and sperm.[1]

  4. Signal molecules

    Cells make signal molecules called eicosanoids from these fats. EPA and DHA compete with arachidonic acid, an omega-6 fat, and this moves the balance toward less inflammatory activity.[1]

  5. Liver

    The liver breaks omega-3 fats down by beta-oxidation. In the liver they also appear to slow the making of triglycerides and to speed up their breakdown, but the exact mechanism is not clear.[4]

Questions about how it worksDoes fish oil reduce inflammation? ›

Benefits and studies

Each row is one benefit. The square shows the grade and the number of studies. Open a row to see the most trusted studies.

StrongModerateWeakNo trials
BenefitMain effects in studiesStudiesPeople

Dose

Large heart trials gave 1 g of fish oil a day, with 840 mg of EPA and DHA, or 4 g a day of a prescription omega-3.[1] The US has set no daily intake for EPA and DHA.[1] EFSA, the EU food safety authority, gives 250 to 500 mg a day as the dietary recommendations for European adults.[17]

Usual dose

For the heart, VITAL and ASCEND gave 1 g of fish oil a day, and REDUCE-IT and STRENGTH gave 4 g of a prescription omega-3.[1] For triglycerides, the American Heart Association reviewed prescription doses of 4 g a day.[19] For blood pressure, a review found the largest fall at 2 to 3 g of EPA and DHA a day.[6] A large pregnancy trial gave 900 mg of omega-3 a day.[8]

Daily need

The US adequate intake covers only ALA, the plant omega-3: 1.6 g a day for men and 1.1 g for women, 1.4 g in pregnancy and 1.3 g in breastfeeding.[1] EFSA advises 100 to 200 mg of DHA a day on top of the adult amount in pregnancy.[1]

Form

A typical 1000 mg capsule holds about 180 mg of EPA and 120 mg of DHA, but products vary widely.[1] Supplements contain natural triglycerides, ethyl esters, re-esterified triglycerides, free fatty acids or phospholipids.[1] Prescription omega-3 products differ from supplements in their make-up and in how they are tested.[3]

When to take

With a meal that contains fat. In a study in men, absorption of EPA from natural fish oil rose from 69% with 8 g of fat to 90% with 44 g.[20] Absorption from ethyl esters rose three-fold, to about 60%.[20]

How long

A 12-week trial saw triglycerides fall.[21] Heart trials lasted 3.5 to 7.4 years.[22][23]

Upper limit

Neither the US nor the EU has set an upper limit.[1][17] EFSA found no safety concern for adults at up to 5 g of EPA and DHA a day, and the US FDA also considers 5 g a day safe.[17][1] Labels in the US should not recommend more than 2 g a day.[1] People who take a blood thinner or have a heart rhythm problem should ask a doctor or pharmacist first.[1][2]

Doses in studies

Most studies gave 2,500 to 5,000 mg a day, for 1 to 3 months.

The counts are study groups. A study group is one set of people in a study who took the same dose. One study can have several groups. These are amounts from studies, not advice for you.

How much a day558 groups in 420 studies
  1. Up to 500 mg27 groups
  2. 500 to 1,000 mg70 groups
  3. 1,000 to 2,500 mg130 groups
  4. 2,500 to 5,000 mg185 groups
  5. 5,000 to 10,000 mg106 groups
  6. 10,000 to 25,000 mg33 groups
  7. 25,000 mg or more7 groups
For how long547 groups in 417 studies
  1. 1 week or less20 groups
  2. 1 to 4 weeks61 groups
  3. 1 to 3 months320 groups
  4. 3 to 6 months95 groups
  5. 6 to 12 months34 groups
  6. More than a year17 groups
Half of the groups took less than 3,000 mg a day. Half took more.Half of the groups took 1,900 to 5,475 mg a day. A quarter took less, a quarter took more.531 of 558 groups took 500 mg a day or more. Do not copy a study dose without a doctor.

Taken together

208 of 597 study groups took fish oil with something else. This shows what studies gave together, not what works together.

  1. Omega-3 Fatty Acids86 studies99
  2. Docosahexaenoic Acid77 studies89
  3. Vitamin E13 studies17
  4. EPA11 studies13
  5. eicosapentaenoic acid (EPA)10 studies12
  6. Eicosapentaenoic Acid9 studies10
  7. Vitamin D10 studies10
  8. Probiotics6 studies6
  9. Vitamin C5 studies6
  10. Whey Protein6 studies6
Bars and numbers count study groups. A companion is shown only when at least 3 different studies gave it.

Protocols

What studies used when they gave fish oil for one goal, or to a group of people.

Each protocol describes what studies used, for one goal or for one group of people. It is not advice for you: talk to a doctor before you start a supplement.

Questions

12 questions in 4 groups. Open one for the short answer; each has its own page with the full answer and sources.

Basics
Is eating fish better than taking fish oil?

For some conditions, the evidence is stronger for seafood than for omega-3 supplements.[3] Many of those findings come from observational studies. They cannot easily separate omega-3 fats from other nutrients in fish, or from the healthier habits of people who eat fish.[1][3]

Trials say little about this question: a Cochrane review found little evidence on the effects of eating fish.[5] Three ounces (about 85 g) of cooked farmed salmon gives 1.24 g of DHA and 0.59 g of EPA, about as much as six typical capsules.[1]

Open the answer with sources ›
Benefits
Does fish oil prevent heart attacks?

It may lower the risk a little. A 2020 Cochrane review of 86 trials found 9% fewer coronary events, with low-certainty evidence, and little or no effect on all cardiovascular events, stroke or death from any cause.[5] In the large VITAL trial, 1 g a day did not significantly lower major cardiovascular events, but heart attacks were 28% lower.[24]

Read the full answer ›
Does fish oil lower triglycerides?

Yes. This is the best supported effect of fish oil. A 2020 Cochrane review found about 15% lower triglycerides, with high-certainty evidence, and a larger effect at higher doses.[5] Prescription omega-3 at 4 g a day lowered very high triglycerides by 30% or more.[19]

Read the full answer ›
Does fish oil during pregnancy lower the risk of preterm birth?

It may, and women with low omega-3 levels seem most likely to benefit.[25] A 2018 Cochrane review found 11% fewer preterm births and 42% fewer early preterm births.[7] A later trial of 5,486 pregnancies found no fall in early preterm birth overall.[8]

Read the full answer ›
Does fish oil help with depression?

The effect is small and uncertain. A 2021 Cochrane review of 34 placebo trials and 1,924 adults with major depression found a benefit of about 2.5 points on a standard scale. A change under 3 points is unlikely to matter in daily life, and the certainty was very low.[11]

A 2019 review suggests that products with at least 60% EPA, at up to 1 g of EPA a day, work better than those high in DHA.[26] Taking fish oil to prevent depression did not help. In 18,353 adults aged 50 or older, 1 g a day for 5.3 years gave 13.9 cases per 1,000 person-years against 12.3 on placebo, a small but significant rise. Mood scores did not change.[27]

Depression can be serious, so anyone who may have it should see a health care provider.[3]

Open the answer with sources ›
Does fish oil improve memory?

Probably not in healthy older adults. A 2020 review of 38 trials and 49,757 people found little or no effect on new dementia, new cognitive impairment or general thinking scores.[13] In people with Alzheimer's disease, trials also found no benefit.[1]

Two groups may gain a little. People with mild cognitive impairment improved in attention, speed and immediate recall in some trials, but this needs confirmation.[1] A 2024 review of 24 trials found a possible gain in executive function, the skills for planning and attention. It was seen in adults aged 40 or older at more than 500 mg a day.[28]

Open the answer with sources ›
Does fish oil help joint pain?

A little, in some people. In osteoarthritis, a 2023 review of 9 trials and 2,070 patients found less pain and better joint function, a small effect.[9] Yet a 2-year trial of 202 people found no extra benefit from 4.5 g a day compared with 0.45 g, and the low dose did better for pain.[29]

In rheumatoid arthritis, a 2024 review of 23 trials found a small, non-significant fall in pain, with very low to low certainty.[10] Another review of 18 trials found fewer tender joints but no significant change in disease scores.[30] Older trials showed lower use of anti-inflammatory painkillers (NSAIDs), but this may reflect weak blinding of the trials.[31][10] People with rheumatoid arthritis should not change prescribed medicines without asking their doctor.[3]

Open the answer with sources ›
Safety
Can fish oil cause atrial fibrillation?

At high doses in people at high heart risk, it can. A 2026 review of 35 trials and 114,592 people found 43% higher odds with more than 1.5 g a day in this group, an extra 0.8 cases per 100 people.[2] It found no significant rise at lower doses or in lower-risk people.[2]

Read the full answer ›
Can you take fish oil with blood thinners?

Studies found little added bleeding risk, but this needs a doctor's review. Most research finds that 3 to 6 g a day of fish oil does not significantly change how warfarin works. Package inserts of prescription omega-3 still call for regular checks of INR, a clotting test.[1]

A 2024 review of 11 trials and 120,643 people found no overall rise in bleeding. Antiplatelet medicines, which stop blood cells called platelets from clumping, did not change the result.[18] High doses of purified EPA were the exception: relative risk rose by 50%, but absolute risk by only 0.6 percentage points.[18] Anyone who takes a blood thinner should ask a doctor or pharmacist before starting fish oil.[1]

Open the answer with sources ›
Does fish oil contain mercury?

Tests have not found it. Seafood contains varying levels of methylmercury, but omega-3 supplements have not been found to contain it, because processing and purification remove it.[1] For one prescription omega-3 product, the starting oil is screened for heavy metals and PCBs.[4]

Pregnant women are advised to choose lower-mercury seafood such as salmon, anchovies, sardines, Pacific oysters and trout.[1] Dietary supplements in the US do not need extensive approval before sale, and their contents can differ from the label.[32]

Open the answer with sources ›
Forms and timing
Which form of fish oil is absorbed best?

Re-esterified triglycerides did best in a 2010 study: 124% of natural fish oil, against 73% for ethyl esters.[33] All forms raise blood EPA and DHA.[1]

Read the full answer ›
Is cod liver oil the same as fish oil?

No. Both contain EPA and DHA, but cod liver oil also contains vitamins A and D, in amounts that vary from product to product.[3] Both vitamins can be harmful in excessive amounts.[3]

People who already take vitamin A or D, or who are pregnant, should ask a doctor or pharmacist before they use cod liver oil.[3]

Open the answer with sources ›

References

  1. Omega-3 Fatty Acids: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. ods.od.nih.gov
  2. Effects of Omega-3 Fatty Acid Treatment on Risk for Atrial Fibrillation: An Updated Meta-Analysis of 35 Trials including 114 592 Individuals. Abuknesha NR, O'Keefe JH, Qian F, Tintle NL, Lin Y, Sun Y, et al. Circ Arrhythm Electrophysiol, 2026. PMID 42517224
  3. Omega-3 Supplements: What You Need To Know. NIH National Center for Complementary and Integrative Health, 2024. www.nccih.nih.gov
  4. Omega-3 Fatty Acids. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury, 2017. PMID 31644217
  5. Omega-3 fatty acids for the primary and secondary prevention of cardiovascular disease. Abdelhamid AS, Brown TJ, Brainard JS, Biswas P, Thorpe GC, Moore HJ, et al. Cochrane Database Syst Rev, 2020. PMID 32114706
  6. Omega-3 Polyunsaturated Fatty Acids Intake and Blood Pressure: A Dose-Response Meta-Analysis of Randomized Controlled Trials. Zhang X, Ritonja JA, Zhou N, Chen BE, Li X J Am Heart Assoc, 2022. PMID 35647665

Review history

  1. 4 Nov 2026CheckedReviewerChecked every claim against its source.
  2. 4 Nov 2026Page createdResearcherFirst edition, written from PubMed and public health sources.