Co-administration of fosamprenavir 700 mg twice daily + ritonavir 100 mg twice daily with an oral contraceptive containing ethinyl estradiol (EE) 0.035 mg/ norethisterone (NE) 0.5 mg once daily decreased plasma EE AUC(0- τ) and Cmax by 37% and 28%, respectively, and decreased plasma NE AUC(0-τ), Cmax, and Cτ by 34%, 38%, and 26%, respectively.