Primidone, carbamazepine, phenobarbital and phenytoin are among the most potent hepatic enzyme inducing drugs in existence. This enzyme induction occurs at therapeutic doses. In fact, people taking these drugs have displayed the highest degree of hepatic enzyme induction on record. In addition to being an inducer of CYP3A4, it is also an inducer of CYP1A2, which causes it to interact with substrates such as fluvoxamine, clozapine, olanzapine, and tricyclic antidepressants, as well as potentially increasing the toxicity of tobacco products. Its metabolite, phenobarbital, is a substrate of CYP2C9, CYP2B6, CYP2C8, CYP2C19, CYP2A6, CYP3A5, CYP1E1, and the CYP2E subfamily. The gene expression of these isoenzymes is regulated by human pregnane receptor X (PXR) and constitutive androstane receptor (CAR). Phenobarbital induction of CYP2B6 is mediated by both. Primidone does not activate PXR.