- Present use or use within 2 months prior to the start of the trial medication of the following drugs: phenytoin, barbiturates, primidone, carbamazepine, oxcarbazepine, topiramate, felbamate, rifampicin, nelfinavir, ritonavir, griseofulvin, ketoconazole, sex steroids (other than pre- and post-treatment contraceptive method) and herbal remedies containing Hypericum perforatum (St John's Wort);