I think Andrew Weil's books are a good common sense source on supplements and he recommends fish oils purified to exclude mercury and PCB's. For counterexample

I think Andrew Weil's books are a good common sense source on supplements and he recommends fish oils purified to exclude mercury and PCB's. For counterexamples, I've noticed a few people taking quite large dosages of B complex vitamins. Dr. Weil notes that mega doses of B3 and B6 can cause problems including irreversible nerve damage. He also recommends against resveratrol supplements, another trendy thing.