Lamotrigine Isoniazid

Phenobarbital, phenytoin, and valproic acid may increase the metabolism of carbamazepine by inducing CYP3A4 carbamazepine may enhance the biotransformation of phenytoin. Concurrent administration of carbamazepine may lower concentrations of valproic acid, lamotrigine, tiagabine, and topiramate. Carbamazepine reduces both the plasma concentration and therapeutic effect of haloperidol. The metabolism of carbamazepine may be inhibited by propoxyphene, erythromycin, cimetidine, fluoxetine, and isoniazid. 

Therefore, plasma concentrations for drugs metabolized by these isoforms will be affected (see Chapter 10). Thus, the addition of phenytoin to an AED regimen can reduce their plasma levels by inducing their CYP3A4 metabolism for carbamazepine, felbamate, lamotrigine, oxcarbazepine, tiagabine, valproate, and zonisamide. Other drugs for which metabolism is induced by phenytoin include methadone, theophylline, warfarin, and oral contraceptives. On the other hand, plasma phenytoin levels are increased by carbamazepine, felbamate, cimetidine, warfarin, chloramphenicol, isoniazid, and disulfiram. Plasma phenytoin levels are decreased by rifampin, antacids, and valproate (free phenytoin levels remain the same). 

Rifampicin markedly increased the clearance of lamotrigine in a pharmacokinetic study. A case report has described a similar finding, and also included some limited evidence suggesting that isoniazid may inhibit lamotrigine metabolism. 

Rifampicin increases the loss of lamotrigine from the body, probably by inducing glucuronidation via UDP-glucuronyl transferases. It was suggested that isoniazid may have inhibited lamotrigine metabolism. ... 

The case report also raises the possibility of an interaction between lamotrigine and isoniazid. If isoniazid is added to or withdrawn from lamotrigine treatment, be alert for the need to adjust the lamotrigine dosage. 

Armijo JA, Sanchez B, Peralta FG, Cuadrado A, Leno C. Lamotrigine interacticxi with rifampicin and isoniazid. A case report. (1996) 18(Si plC),... 

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