Primidone should not be used in patients predisposed to acute intermittent porphyria because enzymes proximal to the deficient enzyme can be stimulated, producing increased amounts of porphyrin precursors. The incidence of fetal malformations is greater in mothers with seizure disorders. Primidone can cause respiratory depression and should be used with caution in patients who have pulmonary disease or status asthmaticus. Patients with hepatic disease can experience toxicity because of reduced metabolism of primidone. Liver-function tests should be assessed before initiating therapy and periodically thereafter.
Complex interactions exist among anticonvulsants.
Because primidone is metabolized to phenobarbital, the drug interactions of phenobarbital also should be considered. In general, phenobarbital can stimulate the metabolism of other anticonvulsants, but it does not appear that other anticonvulsants affect the metabolism of phenobarbital. Other anticonvulsants that can be affected by phenobarbital include carbamazepine, clonazepam, phenytoin, and valproic acid. Primidone may induce its own metabolism. In addition, phenobarbital has been reported to decrease phenytoin metabolism.