Viagra's mechanism of action is quite simple once all elements evolved in the trigger of an erection are known. Basically when men have an erection during a sexual stimulation a gas called nitric oxide is released in the corpus cavernosum (erectile tissues). The oxide is responsible for the activation of the guanylate cyclase enzyme, which results in increasing the level of cyclic guanosine monophosphate (cGMP), causing a smooth muscle relaxation in the corpus cavernosum that allow blood inflow. Sidenafil doesn’t act as a relaxer, but as a catalyst in enhancing the effect of nitric oxide (NO) by suppressing the action of phosphodiesterase type 5 (PDE5) which is the main responsible fot degradation of cGMP in the corpus cavernosum. Whenever sexual stimulation is combined with Viagra, the active ingredient in the pill called Sidenafil causes the increase level of cGMP in the corpus cavernosum resulting in the development and maintenance of an erection.
After administration Viagra is easily and rapidly absorbed in the organism except for the times when the medicine is taken after a high fat meal. The rate of absorption in these cases reduces subsequently with a mean of 60 minutes delay in time.
In clinical trials, Viagra was verified mostly for its effect on the ability of men with erectile dysfunction (ED) to engage in sexual activity and in many cases specifically on the ability to achieve and maintain an erection long enough for satisfactory intercourse. Viagra was tested primarily at doses varying from 25 mg, to 50 mg and 100 mg in over 21 trials divided in real drug control and placebo-control. They were conducted for a period of 6 months using a variety of study design (fix doses, titration, parallel, and crossover). During testing the drug was administered to more than 3,000 patients with age varying from 19 to 87 years, which suffered with various etiologies of ED (organic, psychogenic, and mixed). The statistical results showed a significant improvement in success rate for sexual intercourse in men that took the medicine compared to those that were given the placebo pill.
For the assessment of Viagra's effectiveness a variety of instruments were considered. The fist and most important one was a sexual function questionnaire called the International Index of Erectile Function - IIEF administered during the follow-up of the testing (placebo pill and real drug). Two questions from the IIEF were of major importance for the research the ability to achieve erection sufficient for performing a normal intercourse and the maintenance of erections after penetration. The response was given on a scale ranging from 0 (no reaction to the medicine) to 5 (an erection was reached and maintained all the time). Another mean of measurement of the drug’s efficacy were the daily diaries each patient had to file and the partner’s questionnaire regarding the overall appreciation of the sexual activity.
The test's results were quite impressive as across all trials, above 50% of the patients on taking dosage of 25 mg, 50 mg and 100 mg, respectively, reported an improvement in their erections, compared to 24% on placebo.