Taking health supplements with omega-3 fatty acids can increase the chances of contracting prostate cancer, according to new research.
Omega-3 fatty acids, found naturally in oily fish 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%.
A US team of scientists compared blood samples from 834 men diagnosed with prostate cancer and 1,393 participants without the disease.
High blood concentrations of all three omega-3 fatty acids commonly found in supplements EPA, DPA and DHA were linked to the findings.
Senior report author Dr Alan Kristal told Sky News: "We looked at the marker in blood for the intake of these fatty acids and we found to our surprise that it was associated with increased risk of prostate cancer.
"If you took an omega-3 fatty acid pill, or a fish oil pill every day, you are at the highest risk group."
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".
Further research was needed to uncover the mechanisms that might cause omega-3 to drive prostate cancer, said the researchers.
Nutritionist Nicole Berberian told Sky News: "The fact is that just from this snapshot ... we can't actually say cause of effect, so there is a long way to go before we can say that the cause of this correlation is the actual intake of oily fish.
"So as yet, it is not a cause to panic."
But GP Dr Ellie Cannon said caution was needed when looking at the results of this study.
"It's the type of study where we don't actually know how controlled the trial is and what I mean by that is we don't actually know what other conditions or the other environment that the people within the trial are actually in," she told Sky News.
Dr Cannon said she would not dismiss the study, but advised taking the results with a pinch of salt.
She said it draws attention to issues surrounding supplements.
"I think [the results are] very worrying. It highlights the point that we always think that supplements and natural things are safe and healthy and actually this raises the question, 'what could this actually be causing?'," she said.
"If you don't actually know the proven benefit of taking something do you need to risk taking the risk?"
Omega-3 fish oils are one of the most fashionable and popular supplements on the high street, with Britons reportedly spending around £116m on fish oil supplements.
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 around 41,000 men in the UK are diagnosed with prostate cancer and 11,000 die from the disease.