Decreased plasma concentrations w/ efavirenz, fosamprenavir, cimetidine, esomeprazole, metoclopramide, phenytoin and rifabutin. May increase plasma concentration of ritonavir and atazanavir. May increase risk of neurotoxicity w/ vinca alkaloids (e.g. vincristine, vinblastine). May increase plasma concentrations of tacrolimus, ciclosporin, midazolam, Ca channel blockers (e.g. verapamil, diltiazem, nifedipine, nicardipine, felodipine) and digoxin.