If you want to create a drug that increases blood flow to the penis, there are at least three ways to do it:

1. Increase the amount of nitric oxide produced in the penis
2. Increase the amount of cGMP produced in the penis in response to the nitric oxide
3. Eliminate the PDE in the penis so that the cGMP builds up instead of getting decomposed by the PDE

­Viagra uses method No. 3 -- it eliminates the PDE that is decomposing the cGMP, so cGMP builds up in the penis and has a larger effect on the artery walls. The greater the amount of cGMP the greater the blood flow, and the greater the blood flow the greater the degree of the erection.

Cialis works similarly by blocking PDE(see 2nd source).

Source(s):
http://health.howstuffworks.com/viagra.htm
http://erectile-dysfunction.emedtv.com/cialis/how-does-cialis-work.html


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