Silendafil citrate is a pharmaceutical drug used to treat erectile dysfunction and pulmonary arterial hypertension. It is also useful for preventing and treating high-altitude pulmonary edema associated with altitude sickness. The two most famous trade names for silendafil citrate are Viagra and Revatio, both owned by Pfizer.

Sildenafil (compound UK-92,480) was first synthesized at Pfizer’s research facility in Sandwich, Kent in the United Kingdom. The pharmaceutical chemists were not looking for a way of treating erectile dysfunction; they were trying to find a way of combating hypertension and angina pectoris. (Angine pectoris is a type of chest pain caused by a lack of blood in the heart muscle.)

The first clinical trials took place at the Morriston Hospital in Swansea, Wales. During Phase I of the clinical trials, the drug showed little effect on angina. Instead, it turned out to be a drug capable of inducing penile erections.1

In 1996, Pfizer patented the drug, and in March 1998 it was approved by the United States Food and Drug Administration (FDA) for use in erectile dysfunction. Later that year, pharmacies in the United States began selling Viagra – the first FDA approved oral treatment for erectile dysfunction.2

Viagra pills are blue and diamond-shaped. Pfizer (the name of the manufacturer) is engraved on one side of the pill, while VGR 25 or VGR 50 or VGR 100 is engraved on the other side. VGR is an abbreviation of Viagra, and the number shows the dose of silendafil in a pill. A pill engraved with VGR 25 will thus contain a dose of 25 milligrams of silendafil, and so on.3

Revatio is a white, round, film-coated tablet. On one side of the Revatio tablet, the text RVT 20 is embossed. RVT is an abbreviation of Revatio, and the number 20 indicates that it is a 20 mg dose of sildenafil.4

Viagras primary competitors are Cialis (tadalafil) and Levitra (vardenafil). The FDA approved Levitra for the treatment of erectile dysfunction in August 2003, and Cialis in November that same year.

Viagra works by inhibiting an enzyme named cGMP-specific phosphodiesterase type 5. This enzyme delays degradation of cGMP which regulates blood flow in the penis. Viagra will not just help a man get an erection, it will also help him sustain a erection during intercourse.

In a study published in 2008, sildenafil was shown to improve sexual function in female patients as well.5 The female patients in the study were taking antidepressants. People on antidepressants may experience sexual dysfunction, either as a result of the depression itself or as a reaction to the medicine. According to this study, sildenafil may be a viable treatment for female patients on antidepressant that are experiencing sexual dysfunction.

Sildenafil makes the arterial wall relax, something which leads to decreased pulmonary arterial resistance and pressure. Because of this, silendafil can be used to treat pulmonary arterial hypertension (PAH). When arterial resistance and pressure is decreased, the right-side ventricle does not have to work as hard as before.

Altitude sickness is caused by acute exposure to low partial pressure or oxygen at high altitude. It’s also known as acute mountain sickness since it typically affects mountain climbers. Altitude sickness can progress to high altitude pulmonary edema (HAPE) or high altitude cerebral edema (HACE), two potentially life-threatening conditions. Sildenafil has been show to be useful in preventing and treating HAPE6 7 8.