Evidence-based academic analysis of sildenafil citrate: mechanism, pharmacokinetics, clinical efficacy, and safety profile. Erectile dysfunction is a condition where the penis does not harden and expand when a man is sexually excited, or when he cannot keep an erection. When a man is sexually stimulated, his body's normal response is to increase blood flow to his penis to produce an erection. Oral medicines for ED usually work well and cause few side effects. The four main medicines taken by mouth for erectile dysfunction are: Avanafil (Stendra). Sildenafil (Viagra). Tadalafil (Cialis). Vardenafil. These medicines are called PDE5 inhibitors. Sildenafil (Viagra) is a tablet that's taken by mouth for erectile dysfunction (ED) in adults. It works as a phosphodiesterase-5 (PDE5) inhibitor to help people get an erection or keep an erection that lasts for sex. Viagra relaxes muscles found in the walls of blood vessels and increases blood flow to particular areas of the body. Viagra is used to treat erectile dysfunction (impotence) in men. Learn more about Viagra, including its uses, dosage, side effects, and how to get it. Viagra is used to treat problems relating to male sexual function, particularly problems concerning the ability to achieve and maintain an erection. Learn how to take Viagra safely to treat erectile dysfunction. Swallow a 50 mg dose of Viagra 30-60 minutes before sex. Viagra (sildenafil) is a brand-name prescription drug that’s used to treat erectile dysfunction (ED). Learn about side effects, warnings, dosage, and more. But Viagra is sometimes considered the most effective ED treatment because it’s been used for many years and has a lot of research supporting its effectiveness. Pfizer to lose patent of drug Viagra, Indian companies gear up with copycat versions: Pfizer's patent for the formulation of Viagra, used to treat impotence in men, ends in the US in 2020.