Vertex Pharmaceuticals Inc. said Wednesday that it will more prominently display warnings of possible side effects on the US labels of its blockbuster hepatitis C drug Incivek, following reports of a small number of fatalities.

A known side effect of Incivek is skin rash, and that was noted on the drug’s original label following its US approval in mid 2011. Cambridge-based Vertex said Wednesday that the updating of the US label “follows reports of a small number of fatal skin reactions in patients who continued to receive Incivek combination therapy after a serious skin reaction was identified.”

A Vertex spokesman declined to be specific when asked to quantify the number of fatalities.

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In a statement, the Food and Drug Administration cited data showing that two people had died from serious skin reactions caused by the Incivek combination treatment, including a 47-year-old woman and a 69-year-old man in Japan. (Incivek is used in combination with two other drugs in treating hepatitis C.)

The agency said a total of 112 patients had developed serious skin reactions of two different types. According to Vertex, Incivek has been prescribed to more than 50,000 patients in the United States since it went on the market.

The Vertex spokesman said that the company has been working with the FDA on the label update for the US market.

“Rash and serious skin reactions were part of the Incivek label since approval, and a boxed warning related to serious skin reactions has now been added to the label, giving more prominence to the importance of stopping Incivek if a serious skin reaction occurs,” he wrote in an e-mail.

In a statement, Vertex senior vice president and chief medical officer Robert Kauffman, MD., said: “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.”

In its most recent quarter, Vertex said that Incivek accounted for $254 million of total company revenues of $336 million.

Vertex issued its press release Wednesday morning. In mid-day trading, company shares were trading at $43.52, up 19 cents.

Hepatitis C is a serious liver disease caused by the hepatitis C virus, which is spread through direct contact with the blood of infected people and ultimately affects the liver. Chronic hepatitis C can lead to serious and life-threatening liver problems, including liver damage, cirrhosis, liver failure, or liver cancer.