Outcomes of Other Prescription drugs on VIAGRA Sildenafil metabolism is principally mediated by CYP3A4 (big route) and CYP2C9 (slight route). Therefore, inhibitors of those isoenzymes might lower sildenafil clearance and inducers of these isoenzymes may possibly increase sildenafil clearance.Viagra should only be taken when wanted, about 1 hour bef