Cancer Council Australia chief executive Ian Olver said: ''The reality is that if something is good for you, it doesn't mean that 10 times of it is better. It is unlikely someone would be diagnosed with a deficiency of fish oil. There is a view out there that extra vitamins and antioxidants are good for you. And people take more thinking that more is better.''

Published in the Journal of the National Cancer Institute, the findings confirm a 2011 study that linked high blood concentrations of DHA, an anti-inflammatory fatty acid, to more than double the risk of high-grade prostate cancer. ''The findings suggest that these fatty acids are involved in prostate tumorigenesis and recommendations to increase long-chain omega-3 fatty acid intake, in particular through supplementation, should consider its potential risks,'' the study said.

Professor Peter Howe, a nutrition expert at the University of Newcastle said taking fish oil had ''shown improvements in cardiovascular health, cognitive performance in the aged and reduced inflammation''.

The Heart Foundation recommends adults consume at least 500 milligrams of omega-3 every day from oily fish or fish oil supplements. ''We don't know how much omega-3 people are getting. We should introduce a test to measure omega-3 status when we visit the GP,'' Professor Howe said.

Pros and cons of fish oil supplements