- For patients who are adding ritonavir or efavirenz, concomitant use with drugs that are highly dependent on cytochrome P 450 for clearance, which, in the opinion of the investigators, may lead to the unexpected changes in their concentrations occurring after the new drug is added. Examples include but are not limited to: wafarin-like anticoagulation , amiodarone, astemizole, cisapride, statin class of drugs, gemfibrozil, cyclosporine, ergonavine, ergotamine, methylergonavine, dihydroergotamine, dofetilide, flecanide, quinidine, fusidic acid, methadone, demerol, midazolam, triazolam, phenytoin, dilantin, pimozide, sirolimus, tacrolimus, propafenone, quinupristin, terfenadine, theophylline, rifampin, rifapentene, or rifabutin and illicit substances, including the recreational substance (ecstasy) methylenedioxymethamphetamine