Research review
Does fish oil help prevent cancer, or cause it? What the research shows
A 25,871-person randomized trial (VITAL) found no difference in cancer diagnoses or deaths from 1 g/day omega-3 supplementation. A separate, smaller observational study found higher blood omega-3 levels associated with prostate cancer risk, a finding not replicated by supplementation trials.
The evidence doesn't support fish oil as a way to prevent cancer, and it doesn't support the claim that fish oil causes cancer either. The largest randomized trial to test omega-3 supplementation and cancer directly — 25,871 people, 1 g/day for a median of 5.3 years — found no difference in invasive cancer diagnoses or cancer deaths between the omega-3 group and placebo. A separate, smaller observational study found higher blood omega-3 levels associated with higher prostate cancer risk, a finding that hasn't been replicated in a supplementation trial.
What the evidence doesn't support
It doesn't support fish oil supplementation as reducing cancer risk or cancer deaths — the largest randomized trial on this question found no benefit. It also doesn't support a confident claim that fish oil causes cancer: the one study linking higher omega-3 levels to increased prostate cancer risk measured blood fatty acid levels, not a supplementation trial, and a large randomized trial of actual supplementation found no increase in overall cancer risk. Where these two literatures disagree, the randomized trial — which tests cause and effect directly — carries more weight than the observational one.
What the trials found
The VITAL trial, published in 2019, randomized 25,871 US adults (men 50+, women 55+) to 1 g/day of marine omega-3 fatty acids or placebo, in a factorial design that also tested vitamin D. Over a median follow-up of 5.3 years, invasive cancer was diagnosed in 820 people in the omega-3 group versus 797 on placebo (hazard ratio 1.03, 95% CI 0.93-1.13, not statistically significant), and cancer deaths were also statistically similar between groups (hazard ratio 0.97). This is the largest and most direct test of whether omega-3 supplementation changes cancer risk, and it found no difference in either direction.
Separately, a 2013 case-cohort study within the SELECT trial measured plasma phospholipid fatty acid levels in 834 men later diagnosed with prostate cancer and 1,393 men who weren't. Men in the highest quartile of blood long-chain omega-3 fatty acids had a higher risk of low-grade prostate cancer (hazard ratio 1.44), high-grade prostate cancer (hazard ratio 1.71) and total prostate cancer (hazard ratio 1.43) compared with men in the lowest quartile. This study measured a blood biomarker, not omega-3 supplement intake directly, and it's an association — it doesn't establish that omega-3 intake caused the increased risk. It has also drawn substantial scientific debate since publication, and no randomized supplementation trial, including VITAL above, has found a similar increase in overall cancer risk from actually taking omega-3 supplements.
Questions people ask
Can fish oil cause cancer?
The largest randomized trial of omega-3 supplementation (25,871 people) found no increase in cancer risk from 1 g/day over 5.3 years. A separate, smaller observational study found a blood omega-3 biomarker associated with higher prostate cancer risk, but that's an association in blood levels, not a test of supplementation itself, and it hasn't been confirmed by a randomized trial.
Does fish oil help prevent cancer?
No. The VITAL trial, the largest randomized test of this question, found no reduction in cancer diagnoses or cancer deaths from omega-3 supplementation compared with placebo.
Is fish oil linked to prostate cancer specifically?
One 2013 observational study found higher blood omega-3 fatty acid levels associated with increased prostate cancer risk. That finding is about a measured blood level, not a supplementation trial, and a large randomized trial of actual omega-3 supplementation found no increase in overall cancer risk.
When to talk to a doctor
Cancer screening, a personal or family cancer history, or any new symptom that concerns you are all reasons to talk to a doctor directly rather than relying on a supplement decision. Fish oil is not a treatment or a preventive measure for cancer, and nothing in the evidence above supports using it as one.
Related reading: fish oil benefits: what's established and what isn't and fish oil and heart disease: what the research does and doesn't show.
Sources
- Marine n-3 Fatty Acids and Prevention of Cardiovascular Disease and Cancer In a randomized trial of 25,871 adults, 1 g/day of omega-3 fatty acids for a median of 5.3 years produced no significant difference in invasive cancer diagnoses or cancer deaths compared with placebo.
- Plasma phospholipid fatty acids and prostate cancer risk in the SELECT trial In a case-cohort study measuring blood fatty acid levels (not supplement intake), men with the highest long-chain omega-3 levels had a higher risk of prostate cancer than men with the lowest levels.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.