Narlaprevir is primarily used in the treatment of chronic hepatitis C virus (HCV) infections. It functions as a protease inhibitor, targeting the NS3/4A protease enzyme essential for viral replication. By inhibiting this enzyme, it prevents the virus from multiplying, thereby reducing the viral load in the body. It is often used in combination with other antiviral medications, such as ribavirin and pegylated interferon, to enhance its efficacy and reduce the risk of resistance. This combination therapy has shown significant success in achieving sustained virological response (SVR), which is indicative of a cure for HCV. Narlaprevir is particularly beneficial for patients with genotype 1 HCV, one of the most challenging strains to treat. Its use has contributed to improving treatment outcomes and reducing the progression of liver disease in affected individuals.