However, the dose is usually not more than 80 mg 3 times a day. Sildenafil works on the PDE5 enzyme in the lungs to relax the blood vessels. Do not take it more than once a day. The usual dose is 50mg, when you need it. The dose can be increased to mg or decreased to 25mg depending on its effect. If it is not treated right away, priapism can permanently damage your penis sudden vision loss in one or both eyes.Erectile dysfunction is when a man has difficulty getting an erection or keeping it long enough for sex. It’s also known as ED or impotence. It happens when not enough blood flows to the penis, preventing an erection. The frequency increases with age, affecting about 50% of men over age 40*; however younger men may also experience ED. Examples of Viagra’s more commonly reported side effects include: headache. flushing (temporary warmth, redness, or deepening of skin color) stuffy nose. indigestion (upset stomach) mild vision. 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 .