The concurrent use of
phenytoin and
phenobarbital is normally uneventful. However, changes in
phenytoin levels (often decreases but sometimes increases) can occur if
phenobarbital is added, but seizure
control is not usually affected by this pharmacokinetic interaction.
Phenytoin toxicity following barbiturate withdrawal has been seen. Increased
phenobarbital levels and possibly
toxicity may occur as a result of the addition of
phenytoin to
phenobarbital. Similar interaction may occur with
primidone and fosphenytoin.
Given the unpredictable outcome of concurrent use it would seem prudent to monitor levels if one drug is added to the other or if a dose is changed. Indicators of
phenobarbital toxicity include drowsiness, ataxia or dysarthria.