There was a recent media feeding frenzy over the "revelation" that fish oil could cause prostate cancer. However the problem was that even the people who conducted the study conceded that there was no empirical link! This was because the study was an epidemiological one which attempts to show CORRELATION and not CAUSATION. There is a huge difference (this is discussed entertainingly in the link at the bottom of this article by nutrition expert Dr John Berardi).
Why do these studies make it into the media? Because they are alarmist and attention grabbing - thousands of people consume fish oil so it is fair to say that anyone who values their health would be drawn to this article and (rightly) concerned.
On a more balanced level it is true that fish oil decreases inflamation - so far, so good - however, decreasing inflamation too much is a bad thing. We need a degree of inflamation to catabolise free radicals etc. So in theory TOO MUCH fish oil could ramp down inflamation too much and leave you open to potential problems. However this would have to be Omega 3's, and, as our diet is so badly deficient in this type of fish oil this would be unlikely. Secondly, to rebalance the ratio we just need to consume more Omega 6 and 9, and as these are more than abundant in our diet (too much so, many would argue), once again this would not be a problem.
The truth is, that the huge benefits of fish oil on cardiovascular health, eye health, joint health and brain health far outweigh any negatives, with the caveat being that you must look to purchasing one that is PHARMACEUTICAL GRADE. These are the only type that have been sufficiently purified of PCB's, mercury and other toxins. If you want to see real research on fish oil visit the medical directories website www.pubmed.com for a broad range of excellently researched publications.
For an entertaining article on the difference between types of studies check out this excellent article by Dr John Berardi, a strong advocate of the many benefits of fish oil.