<p>Theophylline should not be used concurrently with other preparations containing xanthine derivatives. The clearance of theophylline is increased by barbiturates, carbamazepine, lithium, phenytoin, rifampicin and sulphinpyrazone and it may therefore be necessary to increase dosage. On the other hand, the clearance of the drug is reduced by allopurinol, cimetidine, ciprofloxacin, corticosteroids, erythromycin, frusemide, isoprenaline, oral contraceptive and thiabendazole and a reduced dosage may therefore be needed to avoid side effects. Theophylline can potentiate hypokalemia resulting from beta-2-agonist therapy, steroids, diuretics and hypoxia, so serum potassium levels should be monitored in such instances.</p>