Decreased theophylline levels w/ barbiturates, charcoal, ketoconazole, rifampicin, smoking & sulfinpyrazone. Increased theophylline levels w/ allopurinol, β-blockers, Ca-channel blockers, cimetidine, OCs, corticosteroids, disulfiram, ephedrine, interferon, macrolides & quinolones. Avoid w/ sympathomimetic agents. Inhibit effect of MAOIs, TCAs.