These medications work by increasing blood flow to your penis, making it easier for you to get and stay hard when you re sexually aroused.
There is an enzyme in the penis called phosphodiesterase that shuts this process off when sex is over.
When a man realizes that he is unable to achieve an erection during sexual stimulation.
This is slowly increased to give the patient a rigid erection for a reasonable time period, which should be no more than 90 minutes and optimally only about 45.