Follow all directions on your prescription label and read all medication guides or instruction sheets. Stepwise increase in alpha-blocker dose may be associated with further lowering of blood pressure in patients taking a PDE5 inhibitor. Ritonavir significantly prolonged the half-life of vardenafil to 26 hours. Other CYP3A4 inhibitors: Although specific interactions have not been studied, other CYP3A4 inhibitors, including grapefruit juice would likely increase vardenafil exposure. Concomitant administration with ritonavir or indinavir substantially increases plasma concentrations of vardenafil.Levitra and its generic version (vardenafil) are available in doses of: milligrams 5 milligrams 10 milligrams 20 milligrams The best dose depends on your: age . Levitra is not a cure for ED and you still need to be aroused for the medication to work. Levitra and its generic version (vardenafil) are available in doses of: milligrams. 5 milligrams. Viagra, Cialis, Levitra, and Stendra are oral medications used to treat erectile dysfunction (ED). You may also know them by their generic names: sildenafil .