Influenza
vaccine can cause a transient moderate (30%) rise in
serum phenobarbital levels. Primidone is metabolised to
phenobarbital and may therefore interact similarly.
Warn the patient to monitor for indicators of
phenobarbital toxicity (drowsiness, ataxia or dysarthria), and take levels if necessary. Levels may still be moderately raised after 28 days. However, most patients seem unlikely to require dosage adjustments.