Fish oil is probably the most popular omega-3 supplement, and for good reason: it contains the fatty acids EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) in forms your body can actually use. This matters because plant-based omega-3 sources like flaxseed contain ALA (alpha-linolenic acid), which your body converts to EPA and DHA at a dismal rate of only 5-15% [1].

If you’re considering fish oil, alternatives include cod liver oil (which also provides vitamins A and D) and krill oil (which some claim has better absorption, though the evidence is mixed).

What is fish oil and why does omega-3 matter?

Omega-3 fatty acids are polyunsaturated fats that your body cannot make on its own, so you need to get them from food or supplements. The main dietary sources are oily fish (salmon, mackerel, sardines), seafood, and to a lesser extent, plant foods like flaxseed and chia seeds.

These fatty acids do more than just provide energy. They’re structural components of cell membranes, particularly in the brain and retina. They also serve as precursors for signalling molecules called eicosanoids, which regulate inflammation, blood clotting, and immune responses [2].

The anti-inflammatory effects of omega-3s are probably their most studied property. Unlike non-steroidal anti-inflammatory drugs (NSAIDs), fish oil doesn’t carry the same risks of stomach ulcers or kidney problems with long-term use [2]. That said, calling it “nature’s most effective anti-inflammatory” is a stretch; the effects are real but modest compared to pharmaceuticals.

Low omega-3 intake has been associated with heart disease, depression, cognitive decline, and various inflammatory conditions. Whether supplementation actually prevents these problems in people with adequate dietary intake is a separate question, and one where the research is messier than supplement marketing suggests.

Researched benefits of fish oil

I’ve gone through the clinical evidence for fish oil’s proposed benefits. What follows is an honest assessment: some findings are promising, others are underwhelming, and quite a few fall into the “interesting but not conclusive” category.

1. May increase resting metabolic rate

Resting metabolic rate (RMR) is the energy your body burns at rest, accounting for the largest portion of daily calorie expenditure. A meta-analysis of 7 randomised controlled trials with 245 participants found that omega-3 supplementation modestly increased RMR, particularly in women and those with a BMI over 25 [3].

I wouldn’t expect dramatic effects here. The increase exists but it’s not going to transform your metabolism.

2. May reduce menstrual pain

Primary dysmenorrhoea (period pain without underlying pelvic disease) affects somewhere between 42% and 95% of women, depending on how you define it. A systematic review found that omega-3 supplementation reduced pain severity [4].

Interestingly, the meta-regression showed that higher doses were less effective, not more. Younger women also responded better. The mechanism likely involves omega-3’s ability to reduce prostaglandin production, which drives uterine contractions.

3. May help rheumatoid arthritis symptoms

Rheumatoid arthritis is an autoimmune disease causing joint inflammation and destruction. A meta-analysis of 20 trials with 717 patients found that omega-3 supplementation (used for more than 3 months) improved multiple disease markers including morning stiffness, joint tenderness, and pain scores [5].

The quality of evidence isn’t stellar, and I wouldn’t suggest replacing disease-modifying drugs with fish oil. But as an adjunct therapy, the data looks reasonable.

4. May benefit polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) affects women of reproductive age and is associated with insulin resistance, irregular periods, and increased cardiovascular risk. A meta-analysis of 9 trials with 591 women found that omega-3 improved insulin resistance, increased adiponectin (a hormone that improves insulin sensitivity), and lowered cholesterol and triglycerides [6].

The trials were relatively short, so long-term effects remain unclear.

5. May reduce some schizophrenia symptoms

This one surprised me. A meta-analysis of 20 double-blind trials involving 1,494 people with schizophrenia found that omega-3 supplementation improved psychopathological symptoms, particularly general psychopathology and positive symptoms (hallucinations, delusions) but not negative symptoms (social withdrawal, lack of motivation) [7].

The benefits were most pronounced in severe cases and with EPA-dominant formulations at doses above 3g daily. I wouldn’t consider this a primary treatment, but it’s an interesting finding.

6. May improve arterial stiffness

Arteries stiffen with age and cardiovascular risk factors, which increases heart disease risk. A meta-analysis of 14 trials found that fish oil reduced pulse wave velocity (a measure of arterial stiffness), particularly at lower doses (≤1.8g/day), shorter durations, and in younger participants [8].

7. May help diabetic blood sugar control

A meta-analysis of 30 trials found that omega-3 supplementation reduced fasting blood glucose and insulin resistance, though it didn’t affect glycated haemoglobin (HbA1c) [9]. If you have diabetes, this might be worth discussing with your doctor, though don’t expect it to replace proper diabetes management.

8. May reduce lupus disease activity

Systemic lupus erythematosus is an autoimmune disease affecting multiple organs. A meta-analysis of 5 trials with 274 patients found that omega-3 supplementation reduced disease activity scores [10]. It might work as an adjunct to standard immunosuppressive therapy.

9. May improve acute lung injury outcomes

In critically ill patients with acute lung injury, a meta-analysis of 6 trials found that omega-3 improved oxygenation and reduced time on mechanical ventilation and in intensive care [11]. The sample sizes were small, so this needs more research.

10. Does not meaningfully reduce muscle soreness

Here’s where expectations meet reality. A meta-analysis of 12 trials found that while omega-3 supplementation showed a statistically significant reduction in delayed-onset muscle soreness after exercise, the effect was not clinically meaningful [12]. Save your money if this is your main reason for taking fish oil.

11. May help perinatal depression

Depression during pregnancy or the first year after birth affects 7-20% of women. A meta-analysis of 8 trials involving 638 women found that omega-3 monotherapy improved depressive symptoms, particularly with higher EPA/DHA ratios (≥1.5), treatment under 8 weeks, and in mild-to-moderate depression [13].

12. May reduce cardiovascular disease risk

A large meta-analysis of 13 trials with over 127,000 participants found that marine omega-3 supplementation (376mg to 4g daily, followed for about 5 years on average) reduced the risk of heart attack, coronary heart disease, and total cardiovascular disease [14]. The risk reduction appeared dose-dependent.

This is probably the most robust evidence for fish oil supplementation, though the absolute risk reduction is smaller than relative risk numbers suggest.

13. Does not clearly help psoriasis

A meta-analysis of 3 trials with 625 participants found no significant reduction in psoriasis severity with fish oil supplementation [15]. If you have psoriasis, the evidence doesn’t support fish oil as a treatment.

14. Does not prevent childhood allergies

A meta-analysis of 5 trials found that omega-3 supplementation in childhood did not reduce the incidence of allergic diseases including asthma, eczema, hay fever, or food allergies [16]. The prevention theory sounded plausible but hasn’t panned out.

15. May improve insulin sensitivity in metabolic syndrome

A meta-analysis of 17 trials found that overall, fish oil didn’t improve insulin sensitivity [17]. However, subgroup analysis showed benefits specifically in people with metabolic syndrome, and short-term supplementation (under 12 weeks) was more effective than long-term.

16. May help depression (especially EPA-dominant formulations)

A meta-analysis of 26 trials with 2,160 adults with clinical depression found that omega-3s improved symptoms [18]. The effect was most pronounced with pure EPA or EPA-dominant formulations (≥60% EPA) at doses up to 1g daily. DHA-dominant formulations didn’t help.

This aligns with the schizophrenia findings. EPA specifically seems to have mood-related benefits.

17. Does not clearly help Alzheimer’s disease

A systematic review of 7 trials found that fish oil supplementation wasn’t significantly helpful for Alzheimer’s symptoms, with modest benefits seen only in mild or early-stage patients on some cognitive scales [19].

I wouldn’t recommend fish oil for established dementia. The research simply doesn’t support it.

18. May improve dry eye symptoms

A meta-analysis of 17 trials with 3,363 patients found that omega-3 supplementation reduced dry eye symptoms, improved tear film break-up time, and increased tear production [20]. If you have dry eyes, this might be worth trying.

19. May improve sperm motility

A meta-analysis of 3 trials with 275 men with infertility found that omega-3 supplementation improved sperm motility [21]. The mechanism may involve maintaining cell membrane integrity and reducing oxidative stress.

20. May benefit non-alcoholic fatty liver disease

A meta-analysis of 18 studies found that omega-3 improved several metabolic markers in non-alcoholic fatty liver disease (NAFLD), including BMI, insulin resistance, cholesterol, and liver enzymes [22]. However, for the more severe form (non-alcoholic steatohepatitis or NASH), omega-3 didn’t improve liver histology.

21. May support foetal brain development

DHA accumulates in the foetal brain and retina during the third trimester. Small trials have found that maternal omega-3 supplementation during pregnancy was associated with better problem-solving scores at 9 months [23] and improved hand-eye coordination at age 2.5 years [24].

22. May reduce preterm birth risk

A meta-analysis of 21 trials with over 10,800 pregnant women found that fish oil supplementation reduced early preterm birth by 22% and preterm birth by 10%, with better birth weight and length [25]. The NHS recommends omega-3 during pregnancy.

23. May reduce hip fracture risk

A meta-analysis of 9 studies with nearly 300,000 participants found that higher fish intake and dietary omega-3 was associated with reduced hip fracture risk [26]. Omega-3s may improve calcium absorption and reduce bone resorption.

24. May slow chronic kidney disease progression

A meta-analysis of 9 trials with 444 people with chronic kidney disease found that omega-3 reduced proteinuria and the risk of progressing to end-stage kidney disease, though it didn’t affect creatinine clearance or glomerular filtration rate [27].

25. May help prevent post-traumatic stress disorder

Research by psychiatrist Yutaka Matsuoka suggests omega-3s may help prevent PTSD by promoting brain-derived neurotrophic factor (BDNF) in the hippocampus, which could reduce the consolidation of fear memories [28]. This is preliminary but intriguing.

26. Associated with lower suicide risk

Epidemiological studies have found that lower fish intake correlates with depression and suicidal behaviour [29], and lower brain DHA levels may increase suicidal intent by 5-6 times. A study of US military personnel found that those with the lowest blood omega-3 levels had a 62% higher suicide rate [30]. A small trial found that 2g daily of EPA+DHA for 12 weeks reduced suicidal thoughts by 45% [31].

These are observational associations and small trials. Correlation isn’t causation, but the pattern is consistent enough to be worth noting.

27. May reduce cancer mortality

Observational studies have found that women with early-stage breast cancer who consumed more omega-3 from fish had 25% lower recurrence rates [32]. Another 15-year study found that breast cancer patients eating more fatty fish had 34% lower all-cause mortality [33]. For prostate cancer, fish intake didn’t affect incidence but reduced mortality by 63% [34].

These are observational studies with all their limitations. They suggest fish consumption matters, though whether supplements provide the same benefit is unclear.

28. May improve acne

A small controlled study in people aged 18-33 with mild-to-moderate acne found that omega-3 (or omega-6 from evening primrose oil) reduced inflammatory pimples by 43% and severity by 29% [35].

How to take fish oil

Studies suggest that taking fish oil with a fat-containing meal increases absorption several-fold [36]. So take it with food, preferably your largest meal.

General recommendations for total omega-3 intake are about 1,100mg daily for women and 1,600mg for men, varying with age and health status. The European Food Safety Authority considers up to 5,000mg of omega-3 supplements daily to be safe, though most people don’t need anywhere near that much.

Eating two servings of oily fish (about 225g total) per week typically provides adequate EPA and DHA. If you don’t eat fish regularly, that’s where supplements become more relevant.

Side effects

For most healthy people, fish oil at appropriate doses (3g or less daily) is well tolerated. Reported side effects include:

Fishy aftertaste or breath
Gastrointestinal upset (acid reflux, nausea, diarrhoea, bloating)
Headache or dizziness
Rash (uncommon)

Taking fish oil with meals and starting with lower doses can minimise digestive issues.

Atrial fibrillation risk: an important caveat

A meta-analysis of 5 trials with over 50,000 participants who had high triglycerides or high cardiovascular risk found that omega-3 supplementation (0.84g to 4g daily) was associated with increased atrial fibrillation risk [37]. This is concerning because these are exactly the populations often recommended fish oil.

If you have elevated triglycerides or cardiovascular risk factors, discuss this finding with your doctor before starting high-dose fish oil.

Safety precautions and contraindications

Contamination concerns: Large predatory fish (tuna, swordfish, shark) may contain mercury and other pollutants. Quality fish oil supplements should be tested for contaminants.
Familial adenomatous polyposis: One case report suggested long-term fish oil use might increase cancer risk in this condition [38]. Use with caution.
Implanted defibrillators: Fish oil may increase arrhythmia risk in people with these devices. Avoid supplementation.
Fish/seafood allergy: While refined fish oil should be protein-free, allergic reactions cannot be completely ruled out.
Immune dysfunction: High-dose fish oil may have immunosuppressive effects (seen in animal studies at human-equivalent doses of 2.2-22g) [39].
Blood pressure medications: Fish oil can lower blood pressure. If you take antihypertensives (captopril, enalapril, losartan, amlodipine, etc.), monitor for excessive blood pressure drops.
Diabetes: High-dose fish oil may raise blood glucose in some people, potentially complicating diabetes management.
Liver disease or cirrhosis: Impaired clotting function plus fish oil’s antiplatelet effects may increase bleeding risk.
Contraceptive pills: May reduce fish oil’s triglyceride-lowering effect.
Orlistat: This weight-loss drug may reduce fish oil absorption.
Anticoagulants and antiplatelet drugs: Fish oil has mild blood-thinning effects. Combining with warfarin, aspirin, clopidogrel, or similar drugs may increase bleeding risk. Aspirin interactions deserve particular attention.
Surgery: Consider stopping fish oil 1-2 weeks before planned surgery due to bleeding concerns.

References

Burdge GC, Calder PC. Conversion of alpha-linolenic acid to longer-chain polyunsaturated fatty acids in human adults. Reprod Nutr Dev. 2005;45(5):581-97. PMID: 9637947
Calder PC. Omega-3 fatty acids and inflammatory processes. Nutrients. 2010;2(3):355-74. PMC3011108
Valisalamizeraj A, et al. Effects of omega-3 polyunsaturated fatty acids supplementation on resting metabolic rate: A systematic review and meta-analysis. Clin Nutr ESPEN. 2021;46:89-95. PMC8716205
Abdi F, et al. Effects of omega-3 supplementation on primary dysmenorrhea: A systematic review and meta-analysis. Nutr Health. 2022;28(1):35-43. PMID: 35059756
Gioxari A, et al. Intake of ω-3 polyunsaturated fatty acids in patients with rheumatoid arthritis: A systematic review and meta-analysis. Nutrition. 2018;45:114-24. PMID: 28965775
Yang K, et al. Effectiveness of omega-3 fatty acid for polycystic ovary syndrome: A systematic review and meta-analysis. Reprod Biol Endocrinol. 2018;16:27. PMID: 29580250
Goh KK, et al. Effect of omega-3 fatty acids on psychopathological symptoms of schizophrenia: A meta-analysis of randomized controlled trials. J Psychopharmacol. 2021;35(3):221-35. PMID: 33586517
Xie X, et al. Fish oil supplementation and arterial stiffness: A meta-analysis of randomized controlled trials. Nutr Metab Cardiovasc Dis. 2021;31(5):1339-48. PMID: 33741211
O’Mahoney LL, et al. Omega-3 polyunsaturated fatty acids favourably modulate cardiometabolic biomarkers in type 2 diabetes: A meta-analysis. Cardiovasc Diabetol. 2018;17:98. PMID: 33480268
Duarte-García A, et al. Effect of omega-3 fatty acids on systemic lupus erythematosus disease activity: A systematic review and meta-analysis. Autoimmun Rev. 2020;19(12):102688. PMID: 33131703
Chen H, et al. Effect of omega-3 fatty acid-based parenteral nutrition on acute lung injury: A meta-analysis. Clin Nutr. 2020;39(8):2492-500. PMC7470437
Kyriakidou Y, et al. The effect of omega-3 polyunsaturated fatty acid supplementation on exercise-induced muscle damage. J Int Soc Sports Nutr. 2021;18:9. PMC7195643
Zhang MM, et al. The efficacy of omega-3 polyunsaturated fatty acids for perinatal depression: A systematic review and meta-analysis. J Affect Disord. 2020;274:1218-27. PMID: 32555188
Hu Y, et al. Marine omega-3 supplementation and cardiovascular disease: An updated meta-analysis of 13 randomized controlled trials. J Am Heart Assoc. 2019;8(19):e013543. PMID: 31567003
Yang SJ, et al. Efficacy of omega-3 polyunsaturated fatty acid supplementation in psoriasis: A meta-analysis. J Dermatolog Treat. 2021;32(5):491-6. PMID: 31805911
Best KP, et al. Omega-3 long-chain polyunsaturated fatty acid supplementation in infancy and allergy-related diseases: A systematic review. Prostaglandins Leukot Essent Fatty Acids. 2020;152:102044. PMID: 31880179
Albert BB, et al. Higher omega-3 index is associated with increased insulin sensitivity and more favourable metabolic profile in middle-aged overweight men. Sci Rep. 2017;7:6723. PMC5496233
Liao Y, et al. Efficacy of omega-3 PUFAs in depression: A meta-analysis. Transl Psychiatry. 2019;9:190. PMC6683166
Burckhardt M, et al. Omega-3 fatty acids for the treatment of dementia. Cochrane Database Syst Rev. 2016;4:CD009002. PMID: 28466678
Giannaccare G, et al. Efficacy of omega-3 fatty acid supplementation for treatment of dry eye disease: A meta-analysis. Cornea. 2019;38(5):565-73. PMID: 30702470
Salas-Huetos A, et al. The effect of nutrients and dietary supplements on sperm quality parameters: A systematic review. Adv Nutr. 2018;9(6):833-48. PMID: 29451828
Yan JH, et al. Omega-3 polyunsaturated fatty acids supplementation and non-alcoholic fatty liver disease: A meta-analysis. Medicine (Baltimore). 2018;97(37):e12271. PMID: 29917092
Judge MP, et al. Maternal consumption of a docosahexaenoic acid-containing functional food during pregnancy: Benefit for infant performance on problem-solving. Am J Clin Nutr. 2007;85(6):1572-7. PMID: 17556695
Dunstan JA, et al. Cognitive assessment of children at age 2.5 years after maternal fish oil supplementation in pregnancy. Arch Dis Child Fetal Neonatal Ed. 2008;93(1):F45-50. PMID: 17185423
Middleton P, et al. Omega-3 fatty acid addition during pregnancy. Cochrane Database Syst Rev. 2018;11:CD003402. PMID: 27012494
Sadeghi O, et al. Dietary intake of fish, n-3 polyunsaturated fatty acids and risk of hip fracture: A systematic review and meta-analysis. Crit Rev Food Sci Nutr. 2019;59(8):1320-33. PMID: 29244536
Elajami TK, et al. Eicosapentaenoic and docosahexaenoic acids attenuate progression of albuminuria in patients with type 2 diabetes and coronary artery disease. Kidney360. 2017;8(3):289-97. PMID: 28226034
Matsuoka Y, et al. Omega-3 fatty acids and PTSD. Prostaglandins Leukot Essent Fatty Acids. 2011;82(4-6):265-9. PMC3045887
Tanskanen A, et al. Fish consumption and depressive symptoms in the general population in Finland. Psychiatr Serv. 2001;52(4):529-31. PMID: 11343534
Lewis MD, et al. Suicide deaths of active-duty US military and omega-3 fatty acid status: A case-control comparison. J Clin Psychiatry. 2011;72(12):1585-90. PMC3259251
Hallahan B, et al. Omega-3 fatty acid supplementation in patients with recurrent self-harm. Br J Psychiatry. 2007;190:118-22. PMID: 17267927
Patterson RE, et al. Marine fatty acid intake is associated with breast cancer prognosis. J Nutr. 2011;141(2):201-6. PMC3021439
Khankari NK, et al. Fish intake and breast cancer survival in a cohort of Chinese women. Cancer Epidemiol Biomarkers Prev. 2015;24(8):1193-8. PMID: 25809414
Szymanski KM, et al. Fish consumption and prostate cancer risk: A review and meta-analysis. Am J Clin Nutr. 2010;92(5):1223-33. PMID: 20844069
Jung JY, et al. Effect of dietary supplementation with omega-3 fatty acid and gamma-linolenic acid on acne vulgaris. Acta Derm Venereol. 2014;94(5):521-5. PMID: 24553661
Lawson LD, Hughes BG. Human absorption of fish oil fatty acids as triacylglycerols, free acids, or ethyl esters. Biochem Biophys Res Commun. 1988;152(1):328-35. PMID: 2847723
Gencer B, et al. Effect of omega-3 fatty acids on the risk of atrial fibrillation: A meta-analysis. Eur Heart J Cardiovasc Pharmacother. 2021;7(5):e69-e70. PMID: 33910233
West NJ, et al. Eicosapentaenoic acid reduces rectal polyp number and size in familial adenomatous polyposis. Gut. 2010;59(7):918-25. PMID: 9879296
Grimm H, et al. Immunomodulation by parenteral lipid emulsions. Am J Clin Nutr. 2006;83(6 Suppl):1541S-1547S. PMID: 24183073