Phenytoin-Theophylline Interaction

Theophylline (1,3-dimethylxanthine) is a constituent of numerous bronchodilator preparations and is widely used in the treatment of patients with obstructive airways disease. Its metabolism is complex and incompletely understood, involving hepatic biotransformation to 3-methylxanthine, 1-methylxanthine, 1,3-dimethyluric acid, and 1-methyluric acid.1 , 2 Only 10 per cent is excreted as unchanged theophylline in urine.3 Phenytoin (diphenylhydantoin) is a widely used anticonvulsant with moderate ability to induce cytochrome P450 mixed-function oxidase activity.4 , 5 In this study, we found that phenytoin given for 10 to 15 days in doses that produced plasma concentrations in the usual therapeutic range decreased the half-life of theophylline and increased . . .