Coadministration has not been studied. Phenytoin is mainly metabolised by CYP2C9 and to a lesser extent by CYP2C19. Atazanavir/ritonavir is metabolized by CYP3A4. Phenytoin is a strong CYP3A4 inducer and may decrease atazanavir/ritonavir exposure. In addition, use with caution as ritonavir may decrease plasma levels of phenytoin due to CYP2C9 and CYP2C19 induction. A dose adjustment of phenytoin may be required. Close monitoring of patient's virologic response should be exercised. Periodic monitoring of atazanavir plasma concentrations should be conducted.