| The following increase clearance and it may therefore be necessary to increase dosage to ensure a therapeutic effect: aminoglutethimide, carbamazepine, moracizine, phenytoin, rifampicin, sulphinpyrazone, barbiturates and Hypericum perforatum. Plasma concentrations of theophylline can be reduced by concomitant use of the herbal remedy St John's Wort (Hypericum perforatum). Smoking and alcohol consumption can also increase clearance of theophylline. The following reduce clearance and a reduced dosage may therefore be necessary to avoid side-effects: allopurinol, carbimazole, cimetidine, ciprofloxacin, clarithromycin, diltiazem, disulfiram, erythromycin, fluconazole, interferon, isoniazid, isoprenaline, methotrexate, mexiletine, nizatidine, norfloxacin, oxpentifylline, propafenone, propranolol, ofloxacin, thiabendazole, verapamil, viloxazine hydrochloride and oral contraceptives. The concomitant use of theophylline and fluvoxamine should usually be avoided. Where this is not possible, patients should have their theophylline dose halved and plasma theophylline should be monitored closely. Factors such as viral infections, liver disease and heart failure also reduce theophylline clearance. There are conflicting reports concerning the potentiation of theophylline by influenza vaccine and physicians should be aware that interaction may occur. A reduction of dosage may be necessary in elderly patients. Thyroid disease or associated treatment may alter theophylline plasma levels. There is also a pharmacological interaction with adenosine, benzodiazepines, halothane, lomustine and lithium and these drugs should be used with caution. Theophylline may decrease steady state phenytoin levels. | |
