Vertex Pharmaceuticals' hepatitis C drug Incivek can cause severe, sometimes fatal, skin rash in some people, the U.S. Food and Drug Administration announced on Wednesday.

The agency added that it is adding a "boxed warning" to the Incivek drug label stating that drug combination treatment must be immediately stopped in patients who develop severe rash.

The drug Incivek is taken with peginterferon alfa and ribavirin (Incivek combination treatment). The drug is given to adult patients who are suffering from chronic hepatitis C with compensated liver disease.

FDA added that patients may develop other symptoms along with rash, like fever, nausea, diarrhea, mouth sores or ulcers, facial swelling, red or inflamed eyes, or swelling or inflammation of the liver (hepatitis). These patients must be given urgent medical treatment.

The announcement came after FDA received two reports of people from Japan who had severe skin reaction after taking the drug combination; one person who developed the skin rash died after continuing using the drug combination.

The black box warning is designed to give prominence to the label warning. Incivek is Vertex's most popular drug; accounting for $254 million of the company's $336 million in revenue, The Boston Globe reported.

"The safety of people taking our medicines is our first priority, and we are committed to ensuring that patients and physicians are aware of the label update to help them use INCIVEK properly. We will continue to educate physicians to follow the rash management plan developed while INCIVEK was in clinical trials and the information contained in the updated label," said Robert Kauffman, M.D., Ph.D., Senior Vice President and Chief Medical Officer at Vertex, in a statement.