Used in the treatment of chronic hepatitis C virus (HCV) infection, specifically genotype 1 and genotype 4. It works by inhibiting the NS3/4A protease, an enzyme essential for viral replication. Simeprevir is typically administered in combination with other antiviral agents such as sofosbuvir or pegylated interferon and ribavirin to increase efficacy and reduce the risk of resistance. It is part of direct-acting antiviral regimens that offer high cure rates, shorter treatment duration, and fewer side effects compared to older therapies. Its use is guided by viral genotype and patient history, including prior treatment response and presence of liver cirrhosis.