Fish Oil Supplements: What the Evidence Really Says — and How to Choose Well

Fish oil is one of the UK's most popular supplements. But does it work, and how do you pick a good one? Here is what the evidence really says.

What is fish oil?

Fish oil contains two omega-3 fats: EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). The British Dietetic Association (BDA) says they are important for the heart, blood vessels, lungs, and the immune and hormone systems, and DHA helps babies' brains and eyes develop (British Dietetic Association, 2024). The best food sources are oily fish such as mackerel, salmon, sardines, herring and trout.

What the research shows

Health area	What the evidence says
Rheumatoid arthritis	The strongest case. A review of 20 trials found omega-3 improved several measures of disease activity, although the trials were generally low quality (Gioxari et al., 2018). Arthritis UK says fish oil can ease joint pain, morning stiffness and tender or swollen joints (Arthritis UK, n.d.).
Heart health	In the VITAL trial, 25,871 US adults took 1g of fish oil a day or a dummy pill for about 5 years. Fish oil did not reduce major heart events or strokes overall, but there were fewer heart attacks (Manson et al., 2019).
Brain health	In 217,122 UK Biobank adults aged 40 to 64, people with the highest blood omega-3 levels had about a 40% lower risk of early-onset dementia. This shows a link, not proof of cause (Sala-Vila et al., 2025).
Osteoarthritis	Weak evidence, with little clear benefit in trials (Arthritis UK, n.d.).
Depression	In over 18,000 older adults, 5 years of fish oil did not prevent depression. The authors said their findings do not support using it for that (Okereke et al., 2021).

What UK experts advise

The BDA notes that omega-3 supplements are not recommended for the general UK population, because the evidence is not conclusive. It advises getting omega-3 from food: 2 portions of fish a week, 1 of them oily (British Dietetic Association, 2024).

The exception is icosapent ethyl (Vazkepa), a purified prescription form of EPA. NICE recommends it only for people with established heart or circulation disease who already take statins, and who have raised triglycerides and cholesterol within set limits (NICE, 2022). It is a medicine, not a shop-bought supplement.

Key insight: food first

Two portions of fish a week, one of them oily, is the simplest, best-evidenced way to get omega-3. A supplement is an option if you do not eat fish.

How to choose a supplement

Check	What to look for
EPA + DHA content	About 450mg of EPA and DHA combined per daily dose (British Dietetic Association, 2024). A 1,000mg fish oil capsule may contain much less, so read the label.
Form	In one study, re-esterified triglyceride (rTG) oil was absorbed better than natural fish oil, and ethyl ester (EE) oil less well (Dyerberg et al., 2010).
Quality	Look for independent testing for freshness and contaminants.
How to take it	With a meal that contains some fat.
Fish body oil, not liver oil	Fish liver oils, like cod liver oil, also contain vitamin A. Too much can cause liver problems (Arthritis UK, n.d.). Do not take cod liver oil or vitamin A supplements if you are pregnant (NHS, n.d.).

Safety

• If you take warfarin, aspirin or other blood thinners, speak to your doctor first, as fish oil can increase the risk of bleeding (Arthritis UK, n.d.).
• Side effects are usually mild, such as stomach upset.
• Fish oil does not replace your prescribed arthritis or heart medicines.

Speak to your GP, pharmacist or a registered dietitian before starting, especially if you have a health condition. Read more in our posts Eating to Ease the Ache and The Joint Pain Swindle.

References

Arthritis UK (n.d.) Fish oils. Available at: https://www.arthritis-uk.org/information-and-support/understanding-arthritis/arthritis-treatments/complementary-and-alternative-treatments/types-of-complementary-treatments/fish-oils/

British Dietetic Association (2024) Omega-3: food fact sheet. Available at: https://www.bda.uk.com/resource/omega-3.html

Dyerberg, J. et al. (2010) 'Bioavailability of marine n-3 fatty acid formulations', Prostaglandins, Leukotrienes and Essential Fatty Acids, 83(3), pp. 137–141. Available at: https://doi.org/10.1016/j.plefa.2010.06.007

Gioxari, A. et al. (2018) 'Intake of ω-3 polyunsaturated fatty acids in patients with rheumatoid arthritis: a systematic review and meta-analysis', Nutrition, 45, pp. 114–124. Available at: https://doi.org/10.1016/j.nut.2017.06.023

Manson, J.E. et al. (2019) 'Marine n-3 fatty acids and prevention of cardiovascular disease and cancer', New England Journal of Medicine, 380(1), pp. 23–32. Available at: https://doi.org/10.1056/NEJMoa1811403

NHS (n.d.) Vitamins, supplements and nutrition in pregnancy. Available at: https://www.nhs.uk/pregnancy/keeping-well/pregnancy-vitamins-and-supplements/

NICE (2022) Icosapent ethyl with statin therapy for reducing the risk of cardiovascular events in people with raised triglycerides (TA805). Available at: https://www.nice.org.uk/guidance/ta805/chapter/1-Recommendations

Okereke, O.I. et al. (2021) 'Effect of long-term supplementation with marine omega-3 fatty acids vs placebo on risk of depression or clinically relevant depressive symptoms and on change in mood scores', JAMA, 326(23), pp. 2385–2394. Available at: https://doi.org/10.1001/jama.2021.21187

Sala-Vila, A. et al. (2025) 'Blood omega-3 is inversely related to risk of early-onset dementia', Clinical Nutrition, 57, 106559. Available at: https://doi.org/10.1016/j.clnu.2025.106559

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Important medical disclaimer

The content provided in this blog post is for educational and informational purposes only and does not constitute professional medical advice, diagnosis, or treatment. Always seek the advice of your GP, psychiatrist or another qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website. Never start, stop or change a prescribed medicine without speaking to your prescriber first.