If you are already being treated with rifabutin, your blood counts and some possible side effects to rifabutin will need to be monitored. • Some medicines used to treat or prevent fits, such as phenytoin, carbamazepine, phenobarbital, primidone. • Efavirenz, which is used to treat HIV infection. • Medicines used to decrease stomach acid such as cimetidine and ranitidine or omeprazole and similar medicines that are called proton pump inhibitors.