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Also known as compound UK-92,480, sildenafil was synthesized in Pfizer's Sandwich, Kent plant. The interesting part is that it was being studied as a treatment option for angina pectoris and hypertension and not erectile dysfunction. Morriston Hospital was the scene of its first clinical trials.
A change of perception
It was in the phase I of the clinical trials that Ian Osterloh, the director of the trials suggested that very little or no effect was seen on the angina, but men taking this medication experienced penile erection. The company took a decision to market it as an erectile dysfunction medication and patented the formula in 1996. This FDA approved drug was launched in the year 1998. Though Peter Dunn and Albert Wood were announced as the inventors of sildenafil by the British press, the claims are disputed by Pfizer.
Availability
Personalities such as soccer legend Pele and United Sates senator Bob Dole have endorsed this prescription drug though T.V commercials. But, this drug can only be prescribed by a doctor. This drug is commonly available in pharmacies and there are also various sites on the internet that help you buy this drug after a process known as "online consultation'; even in this case, it's necessary that you have a prescription for the pill.
Action mechanism of Viagra sildenafil
Cyclic guanosine monophosphate (cGMP) is a substance responsible for erection, so it must be protected from degradation. This is the objective of sildenafil, which belongs to a family of medications known as phosphodiesterase type 5 inhibitors (PDE5). It inhibits the action of PDE5, which breaks down cGMP in the corpus cavernosum (sponge like erectile tissue in the penis).  
Increased levels of cGMP leads to relaxation of the smooth muscles of the arteries; this increases blood flow to the corpus cavernosum. This results in an erection. By inhibiting the action of PDE5, sildenafil helps improve the levels of cGMP in the penis, which results in better and long lasting erections. But, in order for this medication to work you need sexual stimulation. Your body will only release cGMP if there is sexual stimulation
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