Ritonavir, sold under the brand name Norvir, is an antiretroviral medication used along with other medications to treat HIV/AIDS. This combination treatment is known as highly active antiretroviral therapy (HAART). Ritonavir is a protease inhibitor and is used with other protease inhibitors. It may also be used in combination with other medications to treat hepatitis C and COVID-19. It is taken by mouth. Tablets of ritonavir are not bioequivalent to capsules, as the tablets may result in higher peak plasma concentrations. Common side effects of ritonavir include nausea, vomiting, loss of appetite, diarrhea, and numbness of the hands and feet. Serious side effects include liver complications, pancreatitis, allergic reactions, and arrythmias. Serious interactions may occur with a number of other medications including amiodarone and simvastatin. At low doses, it is considered to be acceptable for use during pregnancy. Ritonavir is of the protease inhibitor class. However, it is also commonly used to inhibit the enzyme that metabolizes other protease inhibitors. This inhibition allows lower doses of these latter medications to be used. Ritonavir was patented in 1989 and came into medical use in 1996. It is on the World Health Organization's List of Essential Medicines. Ritonavir capsules were approved as a generic medication in the United States in 2020. Medical uses HIV Ritonavir is indicated in combination with other antiretroviral agents for the treatment of HIV-1-infected patients. Though initially developed as an independent antiviral treatment, it is most commonly used as a pharmacokinetic enhancer, in order to increase the plasma concentrations of other antiretrovirals. Ritonavir is effective in preventing the replication of HIV-1. Protease inhibitors, including ritonavir, effectively block HIV-1 protease, a crucial enzyme in the reproductive cycle of HIV-1. COVID-19 In December 2021, the combination of nirmatrelvir and ritonavir was granted emergency use authorization by the US Food and Drug Administration (FDA) for the treatment of coronavirus disease COVID-19. The co-packaged medications are sold under the brand name Paxlovid. Paxlovid is not authorized for the pre-exposure or post-exposure prevention of COVID-19 or for initiation of treatment in those requiring hospitalization due to severe or critical COVID-19. On 31 December 2021, the UK Medicines and Healthcare products Regulatory Agency (MHRA) approved the same combination "for people with mild to moderate COVID-19 who are at high risk of developing severe COVID-19". Side effects When administered at the initially tested higher doses effective for anti-HIV therapy, the side effects of ritonavir are