Fig. 2. Plasma concentrations of ritonavir (mean ± SD) on the second day of the study in 11 healthy volunteers. On the first day ritonavir 200 mg was given orally three times, and on the second day the ritonavir dose was increased to 300 mg three times. Time 0 h refers to the time of fentanyl injection 2 h after the afternoon dose of ritonavir.