Theophylline: Drug interactions

Certain drugs inhibit theophylline metabolism, thus increasing blood levels and cause toxicity
Ciprofloxacin
Cimetidine
Erythromycin
Allopurinol
Oral contraceptives

Some others increase theophylline metabolism, decreasing blood levels
Phenobarbitone
Phenytoin
Rifampicin

Smoking and consumption of charcoal boiled meat also increase theophylline metabolism.