When did you first begin noticing sexual problems? What special dietary instructions should I follow? Sildenafil comes as a tablet and suspension liquid to take by mouth. What are other possible causes? This medicine also works best when you take it an hour before sex on an empty stomach. Your doctor will tell you not to take sildenafil if you are taking street drugs containing nitrates. Visit Mayo Clinic Men’s Health to get started.Sildenafil (Viagra). This medicine works best when you take it on an empty stomach an hour before sex. The body takes longer to absorb it after a high-fat meal or . It’s a prescription drug that’s used in adult males* with ED. With ED, you have trouble getting or keeping an erection. To find out more about how Viagra is used . VIAGRA (sildenafil citrate) is prescription medicine used to treat erectile dysfunction (ED). VIAGRA is not for women or children. Please see Full Prescribing Information .