Some doctors warn that some popular diet pills could be dangerous. Over-the-counter diet pills known as the "Brazilian pills," claim to be all-natural, but lab tests have found that some variations of them contain illegal amphetamines, NBC 6's Jaylene Rodriguez reported.

A year and a half ago, when the Food and Drug Administration banned the sale of diet supplements containing ephedra, many doctors worried that people who had been taking them would switch to other appetite suppressants of questionable safety.

Some of the weight-loss aids that have quickly emerged to replace the now-banned substance ephedra may carry risks of their own, a small study suggests.