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Concomitant use may lead to lower levels of both drugs. Here is why: Phenytoin induces 3A4, 2B6, 2C9 and Phenobarbital induces 3A4, 2B6 and 2C19. Now Phenytoin is metabolized by 2C9 (major) and 2C19 (minor) and Phenobarbital is metabolized by 2C9 (major), 2C19, and 2E1. You can see that apart from self-induction, both drugs cross induce each others metabolizing P450 isozymes and therefore, may cause a decreased in blood levels of each other. However, note that Phenytoin induces major isozyme (2C9) involved in Phenobarbital metabolism while Phenobarbital induces an isozyme (2C19) which has only minor involvement in Phenytoin's metabolism. I hope that above description would be helpful.

Hello, Are you actively preparing for NAPLEX and/or CPJE? If so and you would like to partner for a serious study group (intense/daily discussion/quizzing via phone or Skype or the like), please contact me. Thank you, BTW, I am not stumped, I just had to select one prefix for the thread. :)

Moricizine is indeed Class 1C. However, it shares some of the properties of Class 1A an 1B drugs. See page 469 of Desk reference of clinical pharmacology By Manuchair S. Ebadi