Follow the directions on your prescription label carefully, and ask your doctor or pharmacist to explain any part you do not understand. Oral medicines for ED usually work well and cause few side effects. Do not stop taking sildenafil without talking to your doctor. Your doctor will tell you not to take sildenafil if you are taking street drugs containing nitrates. This medicine also works best when you take it an hour before sex on an empty stomach.SAVINGS Terms and conditions apply. take any medicines called nitrates, often prescribed for chest pain, or guanylate cyclase stimulators like Adempas (riociguat) for pulmonary . Why is this medication prescribed? Sildenafil (Viagra) is used to treat erectile dysfunction (impotence; inability to get or keep an erection) in men. Drug Images What is Viagra? If you have erectile dysfunction (ED), your doctor may recommend that you take Viagra. It’s a prescription drug that’s used in .