The function of Viagra is relatively simple – it relaxes the muscle and increases the blood flow to the penis. An erection is nothing more than blood flow to the penis. When blood flow is weak so is the erection. If we increase blood flow, we increase the strength of the erection and the ability to maintain it.
The specifics of action involve the binding of nitric oxide to the guanylate cyclase receptors, which increases cGMP levels. This produces a smooth muscle relaxation, which in turn opens the pathway for increased blood flow. It is used to treat certain types of erectile dysfunction
Viagra belongs to the class of drugs known as cGMP Specific Phosphodiesterase type 5 or PED5.
Important note: Viagra as well as other PED5 medications cannot cause or promote an erection without sexual stimulation. They do not influence libido. The subject must be sexually aroused to obtain an erection. In the case of Viagra, the problem for the subject is that the desire is there but the erection is not found. Viagra should reverse this issue if blood flow is the cause.