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, derived from fish oils and lauded for their anti-inflammatory properties, were found to increase the risk of high-grade disease 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%.
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 compared blood samples from 834 men diagnosed with prostate cancer and 1,393 participants without the disease.
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 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."
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".
Omega-3 fish oils are one of the most fashionable and popular supplements on the high street.
They are said to have a plethora of health benefits, including protection against heart attacks and strokes, staving off arthritis, boosting brain power, and preventing behavioural disorders in children.
Each year Britons reportedly spend around £116 million on fish oil supplements. Globally, omega-3 sales run into billions. In 2012, supplements accounted for 10% of the world-wide retail market for omega-3 products, valued at 33 billion dollars (£22 billion).
The new 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.
Men with the highest blood levels of omega-3 fatty acids were significantly more likely to be diagnosed with prostate cancer than those with the lowest levels.
In terms of blood concentration, the difference between the two groups was somewhat greater than the effect of eating salmon twice a week, said lead scientist Dr Alan Kristal, from the Fred Hutchinson Cancer Research Center in Seattle.
"We've shown once again that use of nutritional supplements may be harmful," he said.
Further research is needed to uncover the mechanisms that might cause omega-3 to drive prostate cancer, said the researchers.
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," said Dr Brasky.
Each year around 41,000 men in the UK are diagnosed with prostate cancer and 11,000 die from the disease.