Fish oil (omega-3)

Is fish oil safe?

For most adults, a standard 1 gram capsule a day is low-risk. It supplies about 300 mg of the omega-3s EPA and DHA, far below the 5 grams a day of supplemental EPA and DHA that the European Food Safety Authority judged safe in 2012. Side effects are mostly a fishy aftertaste, burping and loose stools.

Two things keep fish oil out of the top tier. Large trials have linked omega-3 supplements to a higher risk of atrial fibrillation, and independent tests keep finding capsules that are oxidised or contain less omega-3 than claimed.

The atrial fibrillation signal

Atrial fibrillation (AF) is an irregular heart rhythm that raises the risk of stroke. In 2021, researchers led by Baris Gencer pooled seven randomised trials with 81,210 people followed for an average of about five years. Those taking marine omega-3s had more AF, and the effect depended on dose:

Dose of omega-3	Change in AF risk
1 g a day or less	about 12% higher
More than 1 g a day	about 49% higher

Each extra gram added about 11% to the risk. The absolute numbers are modest: in a trial population, a few extra cases per thousand people over several years. But the people most drawn to high-dose fish oil, those with heart disease or high triglycerides, are also those most at risk of AF. In 2023 the European Medicines Agency's safety committee listed AF as a common, dose-related side effect of prescription omega-3 medicines.

Bleeding and blood thinners

Omega-3s make platelets slightly less sticky. At typical supplement doses, trials have not shown a clear increase in serious bleeding. At higher doses the picture changes: in the REDUCE-IT trial of 4 grams a day of prescription icosapent ethyl, serious bleeding occurred in 2.7% of patients against 2.1% on placebo.

If you take warfarin, a DOAC such as apixaban, or clopidogrel, tell your doctor or pharmacist before starting fish oil, and mention it before surgery. Combining it with other supplements that affect clotting, such as turmeric or ginkgo, adds to the effect.

What testing finds inside the capsules

Fish oil is a delicate product. Omega-3 fats oxidise when exposed to heat, light and air, and rancid oil is the most consistent quality problem. A 2015 study of 32 products sold in New Zealand found 83% above the voluntary industry limit for peroxide value, an early marker of oxidation, and many contained less EPA and DHA than labelled. A Canadian study of 171 products found about half exceeded at least one oxidation limit, and a 2019 Australian study reported similar problems. The industry body GOED disputes some of these findings, saying flavourings interfere with the tests.

The health effect of eating mildly oxidised oil is not well established, but it is not what you paid for. Contaminants are less of a worry: molecular distillation removes most mercury and PCBs, and tested products usually fall well within limits.

Choose a product with a USP VerifiedThird-party testingTesting of a supplement by an independent lab to check that it contains what the label says and is free of set contaminants.More in the glossary mark, NSF certification or published IFOS results, which cover oxidation and EPA and DHA content. Store it in the fridge after opening.

Who should be careful

• People with atrial fibrillation or a history of it: discuss dose with your cardiologist.
• People on anticoagulants or antiplatelet drugs, or about to have surgery.
• Pregnant women: purified fish oil is generally acceptable, but cod liver oil contains vitamin A, which can harm a developing baby at high intakes.
• People with a fish allergy: highly refined oil usually contains little protein, but algal omega-3 avoids the question.

Picking a dose

Most people who want omega-3s for general health take about 250 to 500 mg of EPA plus DHA a day, roughly the amount in two portions of oily fish a week. Higher doses (2 to 4 grams) are used to lower triglycerides, but that should be a decision made with a doctor, partly because of the AF signal and partly because prescription versions are better standardised. Read the back label: a "1,000 mg fish oil" capsule usually contains only 300 mg of actual omega-3.

How we scored it

Fish oil does well on dose safety (88) and regulatory record (90). Organ toxicity (82) reflects the heart-rhythm signal, and interactions (82) the mild bleeding effect. Quality (70) is its weakest factor because of repeated findings of oxidised and under-strength products. Compare vitamin D, which has a similar overall profile but fewer quality problems.