Adjustments in phenytoin dose may be needed when phenobarbital is started or stopped. It may be necessary to adjust the dosage of quinidine when phenobarbital is started or stopped. Phenobarbital may reduce theophylline concentrations by enhancing its hepatic metabolism. Valproic acid may increase phenobarbital concentrations by inhibiting hepatic metabolism. The patient should be monitored for phenobartial toxicity, and dosage adjustments may be necessary.
Phenobarbital induces the metabolism of warfarin, which decreases warfarin's effect.
Warfarin level must be carefully monitored and adjusted when phenobarbital is started or stopped.