Gilead Sciences Inc said on Tuesday that the US Food and Drug Administration has approved its hepatitis C drug for patients who had failed to respond to prior treatments.

Vosevi is the first drug to be approved for hepatitis C patients already treated with Sovaldi or other drugs, which inhibit a protein called NS5A, the FDA said in a separate release.

The approval is based on data from two late-stage studies, which evaluated 12 weeks of the drug in previously treated patients, the company said.

However, the approval comes with a boxed warning about the risk of hepatitis B virus being reactivated in patients infected with both hepatitis C and B.

Gilead, whose growth has been fueled by its arsenal of hepatitis C drugs, had warned earlier this year that sales of its high-priced drugs were declining due to fewer patients being deemed eligible for treatment and rising competition.

First-quarter sales of the company's hepatitis C drugs, Sovaldi, Harvoni, and Epclusa, slumped from $4.3 billion a year earlier to $2.6 billion.

Hepatitis C causes inflammation of the liver that can lead to diminished organ function or its failure.

An estimated 2.7 million to 3.9 million people in the US have chronic hepatitis C infection, according to the Centers for Disease Control and Prevention.