To the Editor: Marquis et al. reported that the half-life of theophylline decreased from an average of 10.1 to 5.2 hours, with a subsequent increase in plasma theophylline clearance secondary to concomitant phenytoin therapy (Nov. 4 issue).1 We have also observed subtherapeutic theophylline levels, despite an apparently adequate dosage of theophylline, in patients treated with anticonvulsants. We studied three theophylline-dependent patients with asthma or bronchitis for three days on three separate occasions: when theophylline was administered concurrently with anticonvulsant therapy, one month after anticonvulsant therapy had been discontinued, and one month after anticonvulsant treatment had been reinstituted. After an overnight.
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