Sildenafil (Viagra)

Description

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Sinegra (Viagra, Sildenafil citrate) is a drug used to treat erectile dysfunction (impotence) in males and pulmonary arterial hypertension.

Viagra treats erectile dysfunction and works by relaxing muscles and increases blood flow to particular areas of the body (in this case, the penis, specifically). Its effects last up to four hours. The most common side effects of Viagra are headache, facial flushing, and upset stomach.

As well as treating impotence, Sildenafil Citrate can also be effective in treating Pulmonary Arterial Hypertension (PAH). Similarly to how it acts to relax muscles of the penis, it relaxes arterial walls; therefore, reducing resistance and pressure in the arteries.

Sildenafil Citrate is solely active in relaxing muscles and increasing blood flow to the heart, lungs and penis due to PDE-5, yet vasodilation is not increased in other parts of the body.

Part of the physiological process of erection involves the parasympathetic nervous system causing the release of 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 PDE5 in the corpus cavernosum, resulting in more cGMP and better erections. Without sexual stimulation, and therefore lack of 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.

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 peak effect might be reduced slightly as the plasma concentration will be lowered.

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