Mechanisms for drug interactions among Anti Epileptics
- 1. Inhibition/induction of hepatic enzymes: For example, valproic acid inhibits lamotrigine metabolism, so with initiation of lamotrigine while the patient is taking valproic acid, a slower titration needs to be performed to avoid lamotrigine toxicity. Carbamazepine induces its own metabolism so levels may drop many months after being therapeutic.
- 2. The other main mechanism of drug interaction is through protein binding. Phenytoin, valproate, and tiagabine are highly bound (>90%) to plasma proteins and can compete for binding sites. For example, valproic acid binding can displace phenytoin so that free phenytoin levels increase, resulting in the potential for phenytoin toxicity.