Inhibited bactericidal effect of penicillin. Potentiated effects w/ digoxin, warfarin, carbamazepine, high-dose theophylline. Myalgia, neutropenia & fever w/ vinblastine. Increased serum conc of alfentanil, bromocriptine, hexobarbitone, midazolam, phenytoin, tacrolimus, triazolam, valproate, acenocoumarol. Increased serum levels of simvastatin, lovastatin or other statins. Increased plasma levels of cyclosporin. Ergotism w/ ergot. Increased risk of cardiotoxicity w/ astemizole, terfenadine, cisapride, pimozide. Increased toxicity w/ theophylline. Cardiac toxicity or Torsade de pointes w/ drugs known to prolong QT interval. Interfered w/ serum folate measurements. Verapamil, clozapine.