Drug Interactions. Rifampin can induce metabolism of all PIs and NNRTIs. This can result in more rapid drug clearance and possibly subtherapeutic drug concentrations of the majority of these antiretroviral agents. Rifampin should not be coadministered with the following PIs and NNRTIs: amprenavir, indinavir, lopinavir/ritonavir, nelfinavir, saquinavir, and delavirdine (55). However, it can be used with ritonavir, ritonavir plus saquinavir, efavirenz, and possibly with nevirapine. Rifabutin is an acceptable alternative to rifampin but should not be used with the PI hard-gel saquinavir or delavirdine; caution is advised if the drug is coadministered with soft-gel saquinavir because data are limited. Rifabutin can be administered at one half the usual daily dose (i.e., reduce from 300 mg to 150 mg/day) with indinavir, nelfinavir, or amprenavir or with one fourth the usual dose (i.e., 150 mg every other day or three times a week), with ritonavir, ritonavir plus saquinavir, or lopinavir/ritonavir. When rifabutin is administered with indinavir as a single PI, the dose of indinavir should be increased from 800 mg/8 hours to 1000 mg/8 hours. Pharmacokinetic data indicate that rifabutin at an increased dose can be administered with efavirenz; doses of 450–600 mg/day have been recommended (55). However, available information is limited concerning appropriate dosing if a PI is used concurrently with efavirenz and rifabutin; with such a combination, the rifabutin dose might need to be reduced. Rifabutin can be used without dose adjustment with nevirapine.