A supplement taken by millions for its health benefits may help to trigger aggressive and lethal prostate cancer, research has shown.
Omega-3 fatty acids are derived from fish oils and lauded for their anti-inflammatory properties and used to ease conditions like arthritis.
But they were found to increase the risk of high-grade disease by 71pc.
Taking omega-3 was also associated with a 44pc greater chance of developing low-grade prostate cancer. Overall, the fatty acids raised the risk of all prostate cancers by 43pc.
High blood concentrations of all three omega-3 fatty acids commonly found in supplements EPA, DPA and DHA, were linked to the findings.
Scientists conducting the study in the United States compared blood samples from 834 men diagnosed with prostate cancer and 1,393 participants without the disease.
Over 2,500 men are diagnosed with prostate cancer in Ireland each year. After skin cancer, it is the leading form of the disease in men.
The results add to evidence published in 2011 by the same US team which associated high blood levels of DHA with a doubling of the risk of high-grade prostate cancer.
Co-author Dr Theodore 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."
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 "should consider the potential risks".
Omega-3 fish oils are among the most popular supplements for Irish people and over the years have been promoted for a range of health benefits.
These include protection against heart attacks and strokes, staving off arthritis, boosting brain power, and preventing behavioural disorders in children.
Asked to comment on the study, Dr Harry Comber, Director of the National Cancer Registry in Ireland, said he was loath to give advice to patients to take action on the basis of a single study.
"People should not change their behaviour based on a single study. I would take it with a little bit of caution. It is suggestive but not definitive one way or the other," he added.
There may be confounding factors which influence the findings.
"You have to wait until a fair bit of evidence accumulates and then a review is done to try and summarise the evidence for and against," he explained.
Two leading risk factors for prostate cancer are age and excess weight. The rising incidence here is strongly linked to the increase in testing.