The
serum levels of
theophylline can be markedly reduced by
phenytoin and dosage increases may be needed to maintain therapeutic
theophylline levels. Fosphenytoin, a prodrug of
phenytoin, may interact similarly. Some limited evidence suggests that
theophylline may also reduce
phenytoin levels.
Ideally
theophylline levels should be measured to confirm that they remain within the therapeutic range. Dosage increases of
theophylline of up to 50% or more may be required. The effect of
theophylline on
phenytoin is not established. It may be prudent to monitor
phenytoin levels as well. Separating the oral dosage by 1 to 2 hours appears to minimise the effects of
theophylline on
phenytoin.