They enhance the effects of a chemical the body makes that relaxes muscles in the penis, called nitric oxide. Generic versions of sildenafil, vardenafil and tadalafil are available. It works for up to 36 hours. On 6 May , Pfizer , which manufactures Viagra, told the Associated Press they will begin selling the drug directly to people on its site. Medicines that block an enzyme called CYP3A4. This boosts blood flow and helps you get an erection from sexual activity.Learn about uses, how it works, and more for Viagra (sildenafil). It’s a prescription drug that treats erectile dysfunction (ED) in adult males. Keep in mind that Viagra isn’t approved in the United States to treat PE. And it’s not recommended in current PE treatment guidelines from the American Urological . According to price-comparison site test1.ru, generic Revatio will cost you as little as $ per pill, depending on where you fill your prescription. That’s compared with .