Decreased effect w/ drugs that induce microsomal enzymes (eg, phenytoin, barbiturates, primidone, carbamazepine, rifampicin, & oxcarbazepine, topiramate, felbamate, griseofulvin & products containing St. John's wort), HIV protease (eg, ritonavir) & NNRTIs (eg, nevirapine), increased plasma & tissue conc w/ cyclosporine, decreased plasma & tissue conc w/ lamotrigine. Ethinylestradiol: Decreased conc w/ antibiotics.