Sooriyamoorthy, T. The mechanism of action for ED pills is simple: They inhibit the PDE5 enzyme which regulates the signals responsible for the smooth muscle tissue. These small differences affect the way each medication works, such as how quickly it takes effect and wears off, and the possible side effects. Involve your sexual partner. By controlling the enzyme, sildenafil helps to maintain an erection after the penis is stroked. Your doctor will extensively review your medical history and require you to submit a recent blood pressure reading before providing you with a prescription.Generic Viagra (sildenafil) is available as a , , , and milligram (mg) pill and in liquid form. It’s meant to be taken as needed — about an hour before sex — but shouldn’t be. If you have any questions, contact a member of your care team directly. If you’re a patient at MSK and you need to reach a provider after 5 p.m., during the weekend, or on a holiday, call This information will help you learn to inject medication into your penis. This is called penile injection therapy. Erectile dysfunction (ED) afflicts enough men in the world that none should feel any shame around it — no matter their age. 1 2 And ED medications like Viagra, Cialis, and their much less expensive generics (sildenafil and tadalafil) can make an enormous difference. While most companies provide these medications in pill form, a few .