Nirmatrelvir 150mg combined with Ritonavir 100mg is an oral medication used for treating mild to moderate COVID-19 in adults at high risk of progressing to severe disease. Nirmatrelvir is a SARS-CoV-2 main protease (Mpro) inhibitor that blocks viral replication by preventing the virus from processing proteins essential for its reproduction. Ritonavir, an HIV protease inhibitor, acts as a pharmacokinetic booster by inhibiting the metabolism of nirmatrelvir, thereby increasing its plasma concentration and effectiveness. This combination is taken twice daily for five days, ideally within five days of symptom onset, to reduce hospitalization and death risks from COVID-19.