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Viagra (Sildenafil citrate) is a drug, currently marketed in pill form for treating erectile dysfunction (ED)and pulmonary arterial hypertension (PAH). It was developed by the pharmaceutical company Pfizer. Viagra pills, in 25 mg, 50 mg, 100 mg, 120 mg and 150 mg doses, are blue and diamond-shaped.

How Viagra works?
The mechanism of action of Viagra involves the protection of cyclic guanosine monophosphate (cGMP) from degradation by cGMP specific phosphodiesterase type 5 (PDE5) in the corpus cavernosum. Nitric oxide (NO) in the corpus cavernosum of the penis binds to the receptors of an enzyme called guanylate cyclase which results in increased levels of cGMP, leading to smooth muscle relaxation (vasodilation) of the intimal cushions of the helicine arteries. This smooth muscle relaxation leads to vasodilation and increased inflow of blood into the spongy tissue of the penis causing an erection.
Viagra is metabolised by liver enzymes and excreted by both the liver and kidneys. If taken with a high-fat meal, absorption is reduced; the time taken to reach the maximum plasma concentration increases by around one hour, and the maximum concentration itself is decreased by nearly one-third.

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