Drug Interaction Report

Coadministration with vigabatrin may modestly decrease the plasma concentrations of phenytoin. The exact mechanism of interaction is unknown, although in vitro drug metabolism studies indicate that it may involve induction of CYP450 2C isoenzymes in some patients. A gradual reduction of 16% to 33% in phenytoin plasma levels has been reported in controlled clinical studies of vigabatrin. These changes are unlikely to be of therapeutic significance in most patients, and phenytoin dose adjustments are not routinely required. However, a dose adjustment should be considered if an interaction is suspected.

GENERALLY AVOID: Alcohol may potentiate some of the pharmacologic effects of CNS-active agents. Use in combination may result in additive central nervous system depression and/or impairment of judgment, thinking, and psychomotor skills.

MANAGEMENT: Patients receiving CNS-active agents should be warned of this interaction and advised to avoid or limit consumption of alcohol. Ambulatory patients should be counseled to avoid hazardous activities requiring complete mental alertness and motor coordination until they know how these agents affect them, and to notify their physician if they experience excessive or prolonged CNS effects that interfere with their normal activities.