Flaxseed oil is derived from the seeds for flax, a plant used in making textiles. Unlike fish oil, krill oil and algae oil, flaxseed oil contains only alpha-linolenic acid (ALA) -- no eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA), the fatty acids that confer the health benefits of omega-3s. The human body can convert some ALA to EPA and DHA, but this amount is estimated to be less than 5 percent.
Most of the studies conducted on the health benefits of omega-3 fatty acids have used fish or fish oil, which contain high amounts of EPA and DHA. The few studies that focus on ALA don't show the same beneficial effects. So if you're looking to increase your omega-3 intake for a specific health condition, flaxseed oil shouldn't be your first choice. It is, however, one of the few options available to vegans and vegetarians, it has no fishy taste and it's palatable enough to be added on top of foods such as salads.
More than 900 products on the market include flaxseed oil, but none are USP-verified. For more on USP verification, see What to Look For.