CAMBRIDGE, Mass. (December 19, 2012)—The U.S. Food and Drug Administration is attaching a black box warning to the Vertex Pharmaceuticals hepatitis C treatment Incivek because of a potentially fatal rash.

The FDA says patients taking the pill in combination with two other treatments should stop immediately if they develop a rash that grows worse or comes with symptoms such as a fever, diarrhea or mouth sores.

Patients taking this drug can develop a rash that covers more than half the body.

Vertex Pharmaceuticals Inc. said the drug's labeling had already warned that patients with a serious skin reaction should stop.

The new warning is much more prominent on the label.

Incivek is taken with the pill ribavirin and interferon, which is given by injection.

A black box warning is the most serious form of safety warning.