Coadministration has not been studied. Phenytoin is mainly metabolised by CYP2C9 and to a lesser extent by CYP2C19. Raltegravir is mainly metabolized by UGT1A1. Raltegravir is unlikely to affect phenytoin metabolism. The impact of strong inducers of drug metabolizing enzymes, such as phenytoin, on UGT1A1 is unknown. Coadministration of once daily raltegravir (1200 mg once daily) is not recommended as strong inducers are expected to have a more pronounced effect on raltegravir Cmin when raltegravir is given once daily. The American product label for raltegravir does not recommended coadministration with twice daily raltegravir. However, if coadministration with phenytoin is unavoidable, raltegravir should be used as a twice daily regimen with close monitoring of antiretroviral response. Monitor raltegravir plasma concentrations (when possible).