Used in the treatment of chronic hepatitis C virus (HCV) infection, specifically genotype 1. It is a protease inhibitor that works by blocking the activity of the NS3-4A protease enzyme, which is essential for viral replication. Telaprevir is administered in combination with pegylated interferon and ribavirin to increase effectiveness. It was designed to improve sustained virologic response rates in patients, including those who did not respond well to prior therapies. Its use has largely been replaced by newer antiviral agents with better safety profiles and higher efficacy.