Ritonavir is an antiviral medication primarily used to treat HIV (Human Immunodeficiency Virus) infections. It works by inhibiting the protease enzyme, which is essential for the virus to replicate. By blocking this enzyme, ritonavir helps reduce the amount of virus in the body, allowing the immune system to function more effectively. Originally developed as a standalone treatment, ritonavir is now often used in combination with other antiretroviral drugs to enhance their effectiveness. It is also used in lower doses to boost the levels of other medications, making it an important component in antiretroviral therapy for managing HIV.