Fish Oil Supplement Guide: How to Choose the Right One

Short answer: Read the back label, not the front: a “1,000 mg fish oil” capsule often holds only 300 mg of EPA+DHA. Choose triglyceride (rTG) or phospholipid form, a third-party test (IFOS, USP, NSF or ConsumerLab) and low oxidation values. Compare products on cost per gram of EPA+DHA. Fish oil raises bleeding risk with blood thinners, so ask a doctor first.

To choose a fish oil supplement, check four things: the EPA and DHA content, the chemical form, independent lab testing and freshness. The brain evidence behind fish oil is moderate: memory trials mostly used DHA-rich oils with mixed results, and depression trials found benefit mainly from EPA-rich ones.

This page is about buying and dosing a supplement. For what omega-3s do in the brain, the full trial evidence and food sources, see our guide to omega-3 and brain health.

The trials used specific doses of EPA and DHA, in specific forms, under controlled quality conditions. A typical drugstore capsule may share little with them. One that delivers only 300 mg of EPA+DHA, mostly as poorly absorbed ethyl ester, possibly oxidized and never independently tested, is a different product from the oils used in research.

What to check on the label

A label check takes less than a minute.

• EPA+DHA per serving: Ignore the front of the bottle. Add up the EPA and DHA in the Supplement Facts panel. “1,000 mg fish oil” and “1,000 mg omega-3” are different claims.
• Serving size: Some products reach impressive numbers by defining a serving as two or three capsules. Work out the amount per capsule.
• Form: Look for “triglyceride form”, “rTG” or “re-esterified triglyceride”. If the label says “ethyl ester” or says nothing, assume ethyl ester.
• Testing: Look for IFOS, USP, NSF or ConsumerLab marks. If there are none, check the maker’s website for third-party test results.
• Other ingredients: A short list is best: the oil, the capsule, and perhaps a natural antioxidant (tocopherols, rosemary extract) or lemon oil.
• Expiration date: Omega-3 oils degrade over time. Check that plenty of shelf life remains when the product arrives.

Terms like “pharmaceutical grade”, “triple strength” and “molecularly distilled” are not regulated and don’t guarantee quality. Certificates and published test results do.

EPA+DHA content: the number that matters

The front-label figure, usually “1,000 mg” or “1,200 mg”, is the weight of all the oil in the capsule. That includes other fats.

A standard, unconcentrated 1,000 mg soft gel typically holds about 180 mg of EPA and 120 mg of DHA, so 300 mg of the fatty acids you are paying for. The other 700 mg is fat with no shown cognitive or heart benefit at those doses. To match the 900 mg of DHA a day used in the MIDAS memory trial, you would need several standard capsules a day.

Concentrated products pack 500–900 mg of EPA+DHA into one soft gel, usually in rTG form. They cost more per capsule but less per gram of EPA+DHA.

How to compare value: divide the price by the total EPA+DHA in the bottle. A 30-dollar bottle of concentrated oil with 60 g of EPA+DHA costs 0.50 dollars per gram. A 15-dollar bottle of standard oil with 18 g costs 0.83 dollars per gram, even though it looks cheaper.

Gummies are the weakest option. They usually deliver 50–100 mg of EPA+DHA each, plus added sugar.

Which form absorbs best: triglyceride, ethyl ester or krill

EPA and DHA come in three main forms, and they are absorbed differently.

Triglyceride. In whole fish, omega-3s sit on a glycerol backbone as triglycerides, the form digestion handles best. To make rTG, makers convert the oil to ethyl esters to purify and concentrate it, then attach it back to glycerol.

In a two-week trial, 72 volunteers took about 3.3 g of EPA+DHA a day (Dyerberg 2010). Compared with natural fish oil, bioavailability was 124 percent for rTG and 73 percent for ethyl esters.

Ethyl ester. Many makers stop at the ethyl ester stage because it is cheaper. Ethyl esters are not dangerous, and they do deliver EPA and DHA, but absorption depends on the meal. In a small study in men (Lawson and Hughes 1988), absorption from ethyl esters tripled, to about 60 percent, with a high-fat meal rather than a low-fat one.

In the same study, EPA from triglycerides was absorbed at 69 percent with the low-fat meal and 90 percent with the high-fat meal. If you take ethyl ester oil, take it with a meal that contains fat.

Krill oil. In a crossover trial in 24 healthy adults (Ramprasath 2013), both oils gave 600 mg of omega-3s a day for four weeks. Krill oil raised the omega-3 index more than fish oil.

But a krill capsule often holds 100–150 mg of EPA+DHA, against 500–1,000 mg for a concentrated rTG fish oil. Reaching trial doses takes many capsules and costs much more. Krill oil also contains astaxanthin, a carotenoid antioxidant that helps protect the oil from oxidation.

Third-party testing

The FDA does not require supplement makers to prove their labels are accurate before sale. It acts only after complaints, adverse events or independent investigations. Independent testing fills that gap.

• IFOS (International Fish Oil Standards): The most thorough fish-oil-specific program. It tests EPA and DHA content, oxidation (PV, AV, TOTOX), heavy metals, PCBs, dioxins and furans, rates products on a five-star scale, and publishes lot-by-lot results.
• USP (United States Pharmacopeia): Its Verified Mark means the product contains what the label says, in the stated amounts, without harmful levels of contaminants. Less common for fish oil than IFOS.
• ConsumerLab: Buys products off retail shelves, tests them against their labels and publishes pass/fail reports.
• NSF International: Tests contaminants, label accuracy and manufacturing practice. Its Certified for Sport program suits athletes subject to anti-doping rules.

Without one of these, you are relying on the maker’s own claims. Given how common mislabeling and oxidation problems are in this market, treat testing as a basic requirement.

Oxidation, rancidity and contaminants

Omega-3s have several double bonds in their structure. That makes them prone to oxidation, which breaks them down into peroxides, aldehydes and other reactive compounds.

How common is this? Jackowski (2015) tested 171 over-the-counter supplements sold in Canada; 50 percent exceeded voluntary limits for oxidation markers. Results differ between studies. An industry-funded analysis of 47 products in New Zealand (Bannenberg 2017) found 72–86 percent met industry limits. That contrasts with an earlier report of poor compliance there.

Does it matter for health? A review by Albert (2013) argued that oxidized fish oils may be ineffective or harmful, based mainly on animal studies. It noted that human trials have not reported how oxidized their oils were.

A small randomized trial in 52 Spanish women lasted 30 days (García-Hernández 2013). Less-oxidized capsules lowered blood triglycerides and cholesterol, while highly oxidized capsules had a negative effect on cholesterol. Effects on other outcomes in humans are unknown, but rancid oil is not a neutral substitute for fresh oil.

Oxidation is measured as the peroxide value (PV) and anisidine value (AV), combined into a TOTOX value (2PV + AV). The Global Organization for EPA and DHA Omega-3s (GOED) recommends a TOTOX below 26 for finished products. Good brands publish these values or share them on request. A maker that won’t share them is telling you something.

Fish also build up mercury, PCBs, dioxins and other pollutants, with the highest levels in large predators such as shark, swordfish and king mackerel. Most fish oil comes from small fish (anchovies, sardines, mackerel), but the raw oil still needs purifying.

Molecular distillation, used by reputable makers, removes heavy metals and most organic contaminants to well below safety thresholds. Third-party testing is the only reliable way to confirm a product has been through it.

How much EPA and DHA: doses used in trials

The doses below are what studies used, not a prescription. The evidence behind each is covered in more detail on our omega-3 and brain health page.

DHA for memory. In the MIDAS trial (Yurko-Mauro 2010), 485 healthy adults aged 55 and over with age-related cognitive decline took 900 mg of algal DHA a day or a placebo. After 24 weeks, the DHA group made fewer errors on a visual learning and memory test and did better on verbal recognition memory.

Working memory and executive function did not improve. Larger trials and meta-analyses in older adults without dementia have had mixed results. If memory is your interest, look for a product that lists DHA separately.

EPA for mood. Two meta-analyses of trials in people with depression looked at the EPA share (Sublette 2011, Liao 2019). Formulas with at least 60 percent of their EPA+DHA as EPA reduced depressive symptoms compared with placebo; DHA-dominant formulas did not.

Sublette found benefit across 200–2,200 mg of EPA a day in excess of DHA; Liao found benefit at 1 g of EPA a day or less. A guideline for clinicians from the International Society for Nutritional Psychiatry Research (Guu 2019) covers major depressive disorder. It considers pure EPA, or EPA:DHA ratios above 2:1, effective at 1–2 g of EPA a day.

Depression should be treated with a doctor; fish oil is not a substitute for that treatment. Doses of 1 g a day or more are above the level at which atrial fibrillation risk rose in trials (see Safety below), so discuss them with your doctor.

From food. Two servings a week of salmon, mackerel, sardines or herring provide about 200–500 mg a day of EPA+DHA averaged over the week, depending on the fish (USDA FoodData Central). That can meet the EFSA amount. Salmon and sardines are two of the easiest options.

Algal oil for vegans and vegetarians

Fish don’t make DHA; they get it from microalgae, directly or through the food chain. Algal oil provides preformed DHA straight from cultivated algae.

In a two-week randomized study of 32 healthy adults (Arterburn 2008), algal-oil capsules supplied 600 mg of DHA a day. They raised DHA in plasma and red blood cells as much as the same amount of DHA from cooked salmon.

Newer algal oils often contain EPA too, though DHA usually dominates. For people who avoid animal products, it is the only reliable source of preformed long-chain omega-3s. Plant ALA from flax or chia converts to DHA poorly.

The same quality checks apply. Algal oil usually costs more per gram of EPA+DHA than fish oil, though prices have been falling.

Storage and freshness

• Keep it cool and dark: Heat, light and air drive oxidation. A cabinet away from the stove works; the fridge is better, especially for liquid oil. Freezing capsules is fine and can reduce fishy aftertaste.
• Close the container: Less air means slower oxidation. Use opened liquid oil within the maker’s stated time, typically eight to twelve weeks.
• Smell and taste it: Fresh oil smells mild, slightly oceanic or neutral. A strong fishy, pungent or paint-like smell means it has oxidized. Heavy flavoring can hide this, so bite a capsule if in doubt: rancid oil has a sharp, acrid taste. Persistent acrid fishy burps also point to rancidity.
• Buy from busy sellers: Oil stored for months in a warm warehouse or sunny shelf may be damaged before you open it.

Whatever the form, take fish oil with a meal that contains fat.

Safety

• Bleeding. Fish oil can increase bleeding risk when combined with anticoagulants (such as warfarin) or antiplatelet drugs (such as aspirin or clopidogrel). Tell your doctor or surgeon if you take it, including before an operation.
• Atrial fibrillation. In a meta-analysis of seven large heart trials (81,210 people), omega-3 supplements were linked to a 25 percent higher risk of atrial fibrillation. The increase was larger in trials testing more than 1 g a day (Gencer 2021).
• Upper amounts. EFSA found that supplemental EPA+DHA up to 5 g a day raised no safety concerns for adults, but did not set an upper limit. Higher doses should only be taken under medical supervision.
• Cod liver oil. It contains vitamin A, which can be harmful in high amounts, especially in pregnancy. Pregnant people should not use it without asking their doctor or midwife.
• Side effects. The most common are fishy aftertaste, burping, heartburn and loose stools.
• Interactions and health conditions. Ask a doctor or pharmacist before starting any supplement, especially if you are pregnant, breastfeeding, take medication or have a health condition.

Frequently Asked Questions

Can I get enough omega-3s from diet alone?

Yes, if you eat fatty fish regularly; two servings a week can meet the EFSA amount. If you eat fish rarely or not at all, a supplement can provide EPA and DHA instead. Fish oil has not been shown to prevent cognitive decline.

Should I take fish oil in the morning or evening?

Timing makes little difference. What matters is taking it with a meal that contains fat, and taking it consistently. Pick whichever meal suits you.

Is cod liver oil a good alternative to regular fish oil?

Cod liver oil provides EPA and DHA plus vitamins A and D. Because those vitamins can build up to toxic levels, you can’t take enough cod liver oil to reach high EPA+DHA doses safely. A standard concentrated fish oil gives more dosing flexibility.

Do enteric-coated capsules make a difference?

They release the oil in the small intestine rather than the stomach. That can reduce fishy burps and reflux. They don’t meaningfully change absorption, so they are optional.

Sources

• Albert, B. B., Cameron-Smith, D., Hofman, P. L., & Cutfield, W. S. (2013). Oxidation of marine omega-3 supplements and human health. BioMed Research International, 2013, 464921. doi:10.1155/2013/464921

• Jackowski, S. A., Alvi, A. Z., Mirajkar, A., Imani, Z., Gamalevych, Y., Shaikh, N. A., & Jackowski, G. (2015). Oxidation levels of North American over-the-counter n-3 (omega-3) supplements and the influence of supplement formulation and delivery form on evaluating oxidative safety. Journal of Nutritional Science, 4, e30. doi:10.1017/jns.2015.21

• Bannenberg, G., Mallon, C., Edwards, H., Yeadon, D., Yan, K., Johnson, H., & Ismail, A. (2017). Omega-3 long-chain polyunsaturated fatty acid content and oxidation state of fish oil supplements in New Zealand. Scientific Reports, 7(1), 1488. doi:10.1038/s41598-017-01470-4

• García-Hernández, V. M., Gallar, M., Sánchez-Soriano, J., Micol, V., Roche, E., & García-García, E. (2013). Effect of omega-3 dietary supplements with different oxidation levels in the lipidic profile of women: a randomized controlled trial. International Journal of Food Sciences and Nutrition, 64(8), 993–1000. doi:10.3109/09637486.2013.812619

• Dyerberg, J., Madsen, P., Møller, J. M., Aardestrup, I., & Schmidt, E. B. (2010). Bioavailability of marine n-3 fatty acid formulations. Prostaglandins, Leukotrienes and Essential Fatty Acids, 83(3), 137–141. doi:10.1016/j.plefa.2010.06.007

• Lawson, L. D., & Hughes, B. G. (1988). Absorption of eicosapentaenoic acid and docosahexaenoic acid from fish oil triacylglycerols or fish oil ethyl esters co-ingested with a high-fat meal. Biochemical and Biophysical Research Communications, 156(2), 960–963. doi:10.1016/s0006-291x(88)80937-9

• Ramprasath, V. R., Eyal, I., Zchut, S., & Jones, P. J. (2013). Enhanced increase of omega-3 index in healthy individuals with response to 4-week n-3 fatty acid supplementation from krill oil versus fish oil. Lipids in Health and Disease, 12, 178. doi:10.1186/1476-511X-12-178

• Yurko-Mauro, K., McCarthy, D., Rom, D., Nelson, E. B., Ryan, A. S., Blackwell, A., Salem, N. Jr., & Stedman, M. (2010). Beneficial effects of docosahexaenoic acid on cognition in age-related cognitive decline. Alzheimer’s & Dementia, 6(6), 456–464. doi:10.1016/j.jalz.2010.01.013

• Sublette, M. E., Ellis, S. P., Geant, A. L., & Mann, J. J. (2011). Meta-analysis of the effects of eicosapentaenoic acid (EPA) in clinical trials in depression. Journal of Clinical Psychiatry, 72(12), 1577–1584. doi:10.4088/JCP.10m06634

• Liao, Y., Xie, B., Zhang, H., He, Q., Guo, L., Subramanieapillai, M., … & McIntyre, R. S. (2019). Efficacy of omega-3 PUFAs in depression: A meta-analysis. Translational Psychiatry, 9(1), 190. doi:10.1038/s41398-019-0515-5

• Guu, T. W., Mischoulon, D., Sarris, J., et al. (2019). International Society for Nutritional Psychiatry Research practice guidelines for omega-3 fatty acids in the treatment of major depressive disorder. Psychotherapy and Psychosomatics, 88(5), 263–273. doi:10.1159/000502652

• Arterburn, L. M., Oken, H. A., Bailey Hall, E., Hamersley, J., Kuratko, C. N., & Hoffman, J. P. (2008). Algal-oil capsules and cooked salmon: nutritionally equivalent sources of docosahexaenoic acid. Journal of the American Dietetic Association, 108(7), 1204–1209. doi:10.1016/j.jada.2008.04.020

• Gencer, B., Djousse, L., Al-Ramady, O. T., Cook, N. R., Manson, J. E., & Albert, C. M. (2021). Effect of long-term marine omega-3 fatty acids supplementation on the risk of atrial fibrillation in randomized controlled trials of cardiovascular outcomes: a systematic review and meta-analysis. Circulation, 144(25), 1981–1990. doi:10.1161/CIRCULATIONAHA.121.055654

• Schuchardt, J. P., & Hahn, A. (2013). Bioavailability of long-chain omega-3 fatty acids. Prostaglandins, Leukotrienes and Essential Fatty Acids, 89(1), 1–8. doi:10.1016/j.plefa.2013.03.010