One of the nation's most popular food supplements is coming under sharp criticism from doctors at the Mayo Clinic. It's called 5-HTP and 15 million Americans use it to lift spirits, curb hunger, and fall asleep. But doctors at the Clinic believe it carries significant danger to its users.

Tests on 6 name brands found impurities and toxins that the Clinic believes could lead to serious disease. The FDA confirms the findings and though the supplement hasn't been ordered off shelves, the FDA warns users of its potential dangers.