This medication is taken with or without food an hour before sex. You may opt-out of email communications at any time by clicking on the unsubscribe link in the e-mail. This condition may require prompt medical treatment to prevent serious and permanent damage to your penis. What side effects can this medication cause? How should this medicine be used? · For oral dosage form (tablets): Adults up to 65 years of age—50 milligrams (mg) as a single dose no more than once a day, 1 hour before sexual intercourse. . 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 . The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called .