Sedation, the most frequent undesired effect of phenobarbital, is apparent to some extent in all patients upon drug initiation, but tolerance develops during chronic therapy. Nystagmus and ataxia occur at excessive dosage. Phenobarbital sometimes produces irritability and hyperactiv-ity in children and agitation and confusion in the elderly. Scarlatiniform or morbilliform rash, possibly with other manifestations of drug allergy, occurs in 1—2% of patients. Exfoliative dermatitis is rare. Hypoprothrombinemia with hemorrhage has been observed in the newborns of mothers who have received phenobarbital during pregnancy. As with phenytoin, megaloblastic anemia that responds to folate and osteomalacia that responds to vitamin D occur during chronic phenobarbital therapy.
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