Coadministration has not been studied. Primidone is metabolised by CYP3A4 to the active metabolite phenobarbital. Phenobarbital is an inducer of UGT enzymes and may decrease raltegravir exposure. Alternative anticonvulsants should be considered. 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 US product label for raltegravir does not recommended coadministration with twice daily raltegravir. However, if coadministration with primidone is unavoidable, raltegravir should be used as a twice daily regimen with close monitoring of antiretroviral response. Monitor raltegravir plasma concentrations (when possible).