Fish oil available in over-the-counter supplements might not always be worth the cost.

It is the third most widely used supplement in the United States. But there can be a big difference when it comes to prescription-quality fish oil and the kind found in over-the-counter supplements.

[An excerpt from “Supplements and Safety,” an investigation from FRONTLINE, The New York Times and the Canadian Broadcasting Corporation exploring the hidden dangers of vitamins and supplements, a multibillion-dollar industry with limited FDA oversight.]