Ritonavir is a protease inhibitor that interferes with the virus’s reproductive cycle. Although it was initially developed as an independent antiviral agent, it has been shown to possess advantageous properties in combination regimens with other protease inhibitors. Therefore, it is now more commonly used to boost other protease inhibitors. It is a repurposed drug currently used in combination with other medications to treat coronavirus disease 2019 (COVID-19) in patients at risk of progressing into a severe form. Indications The FDA also authorizes ritonavir for emergency use in Paxlovid – a COVID-19 antiviral treatment which includes nirmatrelvir – for the treatment of mild-to-moderate COVID-19 in adults and pediatric patients (12 years of age and older weighing at least 40 kg) with positive results of direct SARS-CoV-2 viral testing, and who are at high risk for progression to severe COVID-19, including hospitalization or death. Absorption The absolute bioavailability of ritonavir has not been determined. Following oral administration, peak concentrations are reached after approximately 2 hours and 4 hours (Tmax) after dosing under fasting and non-fasting conditions, respectively. It should be noted that ritonavir capsules and tablets are not considered bioequivalent.