It seems that high levels of O3 can increase risk of an aggressive form of prostate cancer according to HuffPo (I couldn't see any hard numbers so although this is an increased risk, total risk may well be quite small - Examine digs further about the quality of this research),
- 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).