New research suggests that omega-3, the health supplement taken by millions of people, may increase the risk of prostate cancer.
Scientists in the United States compared blood samples from more than 2,000 men.
Their study found that the supplement increased the risk of aggressive prostate cancer by 71%.
Taking omega-3 was also associated with a 44% greater chance of developing low-grade prostate cancer.
Overall, the fatty acids raised the risk of all prostate cancers by 43%.
Men with the highest blood levels of omega-3 fatty acids were far more likely to be diagnosed with prostate cancer than those with the lowest levels, the study found.
High blood concentrations of all three omega-3 fatty acids commonly found in supplements, EPA, DPA and DHA, were linked to the findings.
The study involved men participating in the Selenium and Vitamin E Cancer Prevention Trial (Select), which investigated potential ways to reduce the risk of prostate cancer.
No benefit was seen from selenium and an increased number of prostate cancers occurred among men taking vitamin E.
Writing in the online edition of the Journal of the National Cancer Institute, the scientists said the evidence suggested that the fatty acids played a role in prostate cancer development.
People tempted to up their intake of omega-3, particularly by means of supplements, "should consider its potential risks".
The results add to evidence published in 2011 by the same US team that associated high blood levels of DHA with a doubling of the risk of high-grade prostate cancer.
Co-author Dr Thodore Brasky, from Ohio State University, said: "What's important is that we have been able to replicate our findings from 2011 and we have confirmed that marine omega-3 fatty acids play a role in prostate cancer occurrence."
The researchers said further study is needed to uncover the mechanisms that might cause omega-3 to drive prostate cancer.
One potentially harmful effect was the conversion of omega-3 fatty acids into compounds that can damage cells and DNA, they added.
Omega-3 was also thought to contribute to immunosuppression, the dampening down of the immune system.
It was not known to what extent omega-3 might affect the progress of prostate cancer in men who already had the disease.
"It's important to note ... that these results do not address the question of whether omega-3s play a detrimental role in prostate cancer prognosis," Dr Brasky said.
Global sales of omega-3 supplements are estimated at around €2.5 billion annually.