How does Viagra work?

Normal penile erection involves the release of a chemical called nitric oxide from the nerve endings in the penis and the cells that line the blood vessels there. Nitric oxide activates a certain enzyme which results in the production of another chemical called cyclic guanosine monophosphate (cGMP). This chemical relaxes the muscle in the penis, which allows for dilatation of the blood vessels. This increases the blood flow into the penis, which increases the pressure in the shaft. This compresses the outflowing veins, which causes an erection of adequate strength.
Viagra (Sildenafil) works by inhibiting the breakdown of cGMP. When you are sexually stimulated, this pathway of nitric oxide releases with activation of cGMP and dilation of blood vessels is activated. Sildenafil will therefore only be effective if there is sexual stimulation present. It will not cause erections in the absence of such stimulation.

Side effects of Viagra

When it was first launched in the United States in April 1998, trials to date had shown a range of side effects. These have increased in number as new problems come to light with the enormous population of users. The most common side effects include:
1.         headaches
2.         flushes
3.         gastro-intestinal discomfort
4.         nasal congestion
5.         diarrhoea.

Other less common side effects include mild and transient visual disturbances (a blue aura in the vision).                    
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