Brand name: Kamagra
Active substance: Sildenafil citrate
Packaging: blister sheet 4 x 100mg chewable tablets
Shipped from: India

Sildenafil citrate, sold under the names Viagra, Revatio, Vectra and (in the Indian subcontinent) Caverta, is a drug used to treat male erectile dysfunction (impotence) and pulmonary arterial hypertension (PAH), developed by the pharmaceutical company Pfizer.

Part of the physiological process of erection involves the parasympathetic nervous sytem to release nitric oxide (NO) in the corpus cavernosum of the penis. NO binds to the receptors of the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation (vasodilation) in the corpus cavernosum, resulting in increased inflow of blood and an erection.

Sildenafil is a potent and selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum. The molecular structure of sildenafil is similar to that of cGMP and acts as a competitive binding agent of cGMP in the corpus cavernosum, resulting in better erections. Without sexual stimulation and no activation of the NO/cGMP system, sildenafil should not cause an erection. Other drugs that operate by the same mechanism include tadalafil (Cialis?) and vardenafil (Levitra?).

Sildenafil is metabolised by hepatic enzymes and excreted by both the liver and kidneys. If taken with a high fat meal, there may be a delay in absorption of sildenafil and the net effect might be muted slightly as the plasma concentration will be lowered. Some reports have claimed that sildenafil causes enhanced sexual pleasure for women by increasing blood flow to the sexual organs.

Pulmonary hypertension
Studies have shown that apart from in erectile dysfunction, sildenafil citrate is also effective in the rare disease pulmonary arterial hypertension (PAH). It appears to relax the arterial wall, decreasing the pressure on the lung vasculature and improving symptoms of right-sided heart failure. Due to the distribution of PDE-5, which is primarily in the smooth muscle of the lungs and penis, sildenafil acts selectively in both these areas without inducing vasodilation in other areas of the body. As a result of these studies, Pfizer submitted an additional registration for sildenafil with the FDA, and was approved for this indication in June 2005. Presumably to avoid confusion with Viagra, the preparation is named Revatio and the 20 milligram tablets are white and round. Sildenafil follows bosentan and prostacyclin as therapies for this condition.

Raynaud's phenomenon
In 2005, Dr. Roland Fries and colleagues reported that sildenafil cut the frequency of Raynaud's phenomenon attacks, reduced their duration by roughly one half, and more than quadrupled the mean capillary blood velocity. This was a double-blind, placebo-controlled crossover trial and the patients had both the primary and secondary forms and had all discontinued the more conventional treatments for this (Fries et al 2005). It had earlier been reported to be effective for Raynaud's in a 65-year-old woman with scleroderma and pulmonary hypotension (Rosenkranz et al 2003); (Rosenkranz et al 2004), and in nine women and one man treated by a Dr. Jack R. Lichtenstein of Annapolis, Maryland over a three-year period (Lichtenstein 2003).

Before taking any prescription medicine, it is important that you consult your doctor!