To the Editor: The recent information that methylphenobarbitone (60 mg tablets) will be unavailable after 1 January 2003 has caused anxiety in patients with epilepsy previously treated satisfactorily with this drug. The suggested substitution of phenobarbitone, primidone or a newer antiepileptic agent seems appropriate. However, patients and perhaps practitioners may assume, as my patients have, that phenobarbitone and primidone are equivalent to methylphenobarbitone on a milligram-for-milligram or a tablet-for-tablet basis. This may not be so. The equivalence is close to 30 mg of phenobarbitone for 60 mg methylphenobarbitone, and probably around 200 mg of primidone for 60 mg of methylphenobarbitone.1 Plasma phenobarbitone concentrations should be checked before and after any changeover.
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