Nirmatrelvir drug for Covid-19, molecular model. Nirmatrelvir is an antiviral medication developed to treat Covid-19. The drug is a protease inhibitor that acts on the spike protein of the SARS-CoV-2 virus. Paxlovid, a combination drug of nirmatrelvir and the antiviral ritonavir is currently in Phase III testing. Atoms are colour-coded: carbon is black; hydrogen is white; nitrogen is blue; oxygen is red.