Sildenafil is available as oral tablets at doses of 25 mg, 50 mg, and 100 mg. Patients should take sildenafil approximately one hour before sexual activity. In some men, the onset of action of the drug may be as early as 11-20 minutes. It's best for men to take sildenafil on an empty stomach for best results since absorption and effectiveness of sildenafil can be diminished if it is taken shortly after a meal, particularly a meal that is high in fat. Sildenafil and the other PDE5 inhibitors don't cause an immediate erection. Sexual stimulation is necessary for these medications to work.
Getting (and maintaining) an erection requires a surprising amount of things to go right. You have to get aroused, then pass that signal from your brain, through your nerves and hormones, to your blood vessels and muscles before an erection can even happen. If one thing goes wrong in that complicated exchange between your cardiovascular, and nerve system, and your hormone levels, blood vessels, and even your mood the result is usually erectile dysfunction. In other words, getting an erection is hard.