Tadalafil Citrate

How was the compound sildenafil citrate (Viagra) and what has been found it extracts?
Are Tadalafil Citrate (Cialis) and vardenafil HCl (Levitra) extracted from the same plant source that Viagra is it?

It was not necessarily directly extracted from a plant source. Scientists to study systematically the pharmaceutical molecules per million. Some of which were taken from plant sources and others where not. Most of them are changes from molecules previously studied. They can extract a substance from a plant, to analyze its molecular composition, and then find a molecule that shows promise and warrants further study. Then, they may find that the molecule is not desirable for any reason, they could slightly change the molecule, then test the new molecule which has been formed, and so on. They literally study and throw millions of molecules before finding one that shows a useful application medical. With computers in this process is much faster than it was.
In the case of Viagra, sildenafil citrate was originally developed to treat ischemic heart disease. In studies of humans, they found that sildenfil has not shown the promise they had hoped, but by collecting feedback from patients about side effects, they have fallen into the "side effect" of a hard erection in patients who had not had a "good erection" during the years. They decided to continue the review and found it very effective for this purpose. They have realized that Erections are controlled by an enzyme in the body called phosphodiesterase. Variations of this enzyme are found throughout the body do different things, and are numbered for classification purposes. Phosphodiesterase in the penis called PDE5. Other enzymes such as PDE11A are found in skeletal muscle. In short, the PDE5 prevents an erection swallowing nitric oxide. Nitric oxide is released during the stimulation to induce erection, but impotent men have a high threshold of PDE5 for it to happen. Viagra, Cialis, Levitra and all work temporarily inhibiting PDE5, nitric oxide is released during excitation and is authorized to cause erections. The differences between the drugs is due to levels at which other PDE are inhibited. Viagra also inhibits PDE3, which is in the eye, for color vision, so you can sometimes have a bluish tint to your vision temporarily. Cialis and inhibits PDE11A, which causes an increase in the incidence of muscle pain. The drugs also differ as to their speed of action, and the half-life.