Major
Phenytoin × Theophylline
Antiepileptic, hydantoin derivative (sodium channel blocker)×Methylxanthine
Mechanism
Phenytoin induces CYP1A2. Theophylline falls 30-50%.
Symptoms
Return of obstructive symptoms.
Management
Theophylline level monitoring with dose adjustment.
Check the full regimen, not just this pair
Opens the checker with these two drugs prefilled. Add the rest of the regimen and recompute additive risks.