CAMBRIDGE, Mass. (AP) — The Food and Drug Administration is attaching a black box warning to the Vertex Pharmaceuticals hepatitis C treatment Incivek over 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 like a fever, diarrhea or mouth sores.

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

Vertex Pharmaceuticals Inc. says 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.