Kamagra (sildenafil citrate) is one of a family of medications referred to as PDE-5 inhibitors.
Other PDE-5 inhibitors are:
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tadalafil (Apcalis)
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vardenafil (Auvitra)
All PDE-5 inhibitors work in the same way. It should be noted that PDE-5 inhibitors will only produce and erection with the aid of sexual stimulation.
Sexual stimulation causes Nitric Oxide (NO) to be released within the walls of the penis. NO acts as an activation agent for the guanylate cyclase enzyme. This enzyme increases the level of cyclic guanosine monophosphate or cGMP in the walls of the penis.
cGMP relaxes the blood vessels supplying blood into the penis which are naturally intended to limit blood-flow. This relaxation allows blood to enter the penis faster and an erection occurs.
PDE-5 inhibitors slow down the re-absorbsion of cGMP into the body and aim to keep the cGMP levels elevated so an erection can be achieved and maintained.
Now you can see why sexual stimulation is necessary before Kamagra can enhance a man's erection.