Vertex Pharmaceuticals Inc. will add the FDA’s most serious warning to the US label of the Cambridge company’s best-selling drug, Incivek, following reports that at least two patients died from severe skin reactions after taking it in combination with two other hepatitis C treatments.

The so-called black box warning — it is bordered in black — is designed to give more prominence to existing label language and to the way Incivek-related skin rashes should be treated.