But how much of this is backed by science, and when might fish oil supplements actually be worth taking? Fish oil comes from oily fish such as salmon, sardines, tuna, herring and mackerel. A “1,000 mg” fish oil capsule usually refers to the total oil weight of the oil, not the active omega-3 content. For otherwise healthy people, the evidence that standard fish oil supplements prevent heart attacks or strokes is much less convincing. If you don’t eat fish, a fish oil supplement (or algal oil if you’re vegetarian or vegan) can help you meet your omega-3 needs.