Omega-3 fish oil supplements may raise the risk of cancer in women who have had a heart attack or other cadiovascular problems, but no similar link was found in men, a new study suggests.

Researchers, reporting in the Archives of Internal Medicine, said women have a higher cancer risk after five years of supplementation, but men with a history of cardiovascular problems did not appear to face any increased risks from the supplements.

Researchers from INSERM – the Institut National de la Santé in Paris – monitored 2,500 people from 2003 to 2009 who had survived a heart attack, stroke or angina in the prior 12 months. The intent of the study was to determine if vitamins B6 and B12 or omega-3 supplements could help prevent a future stroke or heart attack.

The researchers concluded: "Neither B vitamin supplementation nor omega-3 …supplementation has a significant effect on the occurrence of cancer in men. However, women receiving omega-3 fatty acids tended to present a higher risk of cancer.”

The reason for the difference was unclear, researchers said, but they speculated the supplements may interact negatively with estrogen hormones.