Two popular diet pills will soon come with a new warning that they could cause severe liver damage in rare cases.

The Food and Drug Administration added warning labels Wednesday to the drugs alli, which is sold over-the-counter, and the prescription version Xenical.

An estimated 40 million people have used the drugs alli and Xenical, which both contain the ingredient orlistat. That ingredient has been linked to 13 cases of damaged livers.

The FDA urged doctors and patients to watch for signs of liver injury, like itching, yellow eyes and skin and loss of appetite.

Both of the drug's manufacturers defended they safety of their product Thursday.

GlaxoSmithKline, which produces alli, said they are "committed to ensuring that consumers and physicians understand the safety profile."

Xenical's maker, Roche, said the "safety profile of Xenical is based on more than 10 years of clinical experience and more than 36 million patients worldwide have received Xenical."