Primidone is a congener of phenobarbitone and is similar in action to it, but is much less potent. One of the two active metabolites of primidone is in fact phenobarbitone, the other being phenylethylmalonamide (PEMA).
Common adverse effects include sedation, vertigo, nausea, ataxia, diplopia, and nystagmus. Serious adverse effects are rare and comprise leukopenia, thrombocytopenia, SLE, lymphad-enopathy, and psychotic reactions.
Primidone overdose usually presents with coma and loss of deep tendon reflexes, nystagmus, strabismus, ankle and knee clonus, and positive Babinski, Hoffman, and Chaddock signs. In severe toxicity, massive crystalluria occurs with passage of hexagonal crystals in urine.
Primidone levels of 5 to 15 mcg/ml may be considered therapeutic. Levels greater than 15 mcg/ml are associated with toxicity, and levels of 70 to 80 mcg/ml are associated with the development of crystalluria.
Treatment is on the same lines as for phenobarbitone poisoning.