Drug Interaction Report

Case reports suggest that nicotinamide may increase carbamazepine levels. The mechanism may be related to nicotinamide-induced inhibition of carbamazepine metabolism. If these drugs must be used together, close observation for clinical and laboratory evidence of carbamazepine toxicity is recommended.

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

In a small, randomized, crossover study, the administration of carbamazepine with grapefruit juice (compared to water) increased plasma drug concentrations by approximately 40%. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits.

MANAGEMENT: Patients receiving carbamazepine should be advised to avoid or limit consumption of alcohol. Given the drug's narrow therapeutic index, patients receiving carbamazepine therapy should preferably avoid the regular consumption of grapefruits and grapefruit juice to prevent any undue fluctuations in plasma drug levels. Patients should be advised to report signs of carbamazepine toxicity (nausea, visual disturbances, dizziness, or ataxia) to their physicians.

GENERALLY AVOID: Ethanol can exacerbate the cutaneous flushing that is a common side effect of niacin. At least one case of delirium and lactic acidosis has been reported with coadministration of these drugs, although data are limited.

MANAGEMENT: Coadministration should probably be discouraged, particularly since chronic consumption of large amounts of alcohol is associated with hyperlipidemia.

A single case has been reported in which cutaneous flushing and tremors were noted in a woman who was taking niacin while wearing a nicotine patch. The mechanism is suspected to be synergistic cutaneous vasodilatory effects. The clinical significance of this possible interaction is not known.