Sildenafil may cause side effects. They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. What special dietary instructions should I follow? Do not mix the liquid with other medications or add anything to flavor the medication. These include itraconazole Sporanox, Tolsura , ketoconazole Extina, Ketozole, others and ritonavir Norvir.VIAGRA (sildenafil citrate) is prescription medicine used to treat erectile dysfunction (ED). VIAGRA is not for women or children. Please see Full Prescribing Information and .  · Follow all directions on your prescription label and read all medication guides or instruction sheets. Revatio is usually taken three times each day, about 4 to 6 hours . Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in .