Oral ED medicines might not work or might be dangerous if you also take any of the following: Nitrate medicines. One study has shown, however, that Viagra may improve premature ejaculation , meaning you will last longer during sexual activity. These should prompt you to seek immediate medical attention, as not doing so could lead to penile tissue damage and potentially a permanent loss of potency. Viagra, containing the active ingredient sildenafil, belongs to a class of drugs called phosphodiesterase PDE -5 inhibitors. Summary Viagra is a PDE-5 inhibitor, a pill taken by mouth that helps you get an erection and keep it longer. Taking Viagra one hour before sexual activity is recommended for most people, but you can take it anywhere from 30 minutes to four hours prior.Viagra is a drug that works by helping men maintain an erection. Without touch to the penis, like that experienced in sexual intercourse, an erection will not occur, . Viagra dosage. Most men start by taking one Viagra 50 mg tablet. (This is the strength of over-the-counter Viagra Connect tablets.) Some men will need to start with a lower dose . As a result, many will turn to Viagra (sildenafil) as a quick, effective solution for their ED. Viagra can last for about four hours or more in some people and should be .