Viagra (sildenafil citrate) is an oral treatment for erectile dysfunction (ED). The following steps explain the mechanism in which Viagra and an erection work.

* The dose is taken (swallowed) orally.
* Viagra (sildenafil citrate) is then quickly absorbed and introduced to the circulatory system (bloodstream). Viagra usually takes 30-60 minutes to get fully active and then stays effective for up to 4 hours.
* When Viagra (sildenafil citrate) reaches the penis it blocks/inhibits an enzyme in the penis called PDE5 (phosphodiesterase) – PDE5 is responsible for ending an erection and bringing the penis back to a flaccid state.
* If sexual stimulation (physical or mental) occurs during the time of action of Viagra, the brain starts sending signals to the penis to begin the production of cyclic guanosine monophosphate (cGMP).
* cGMP relaxes muscles in blood vessels and arteries and allows them to expand and carry more blood to the penis as long as the stimulation signals from the brain keep coming.
* The diminution of PDE5 (an enzyme that decomposes cGMP) in the penis allows blood vessels to remain relaxed during sexual arousal.
* The increased blood flow to the penis causes it to become and remain fully erect during arousal.
* When sexual arousal ends, the brain stops sending its signals and the production of cGMP stops.
* Blood vessels start to contract and get back to the normal state, blood flow to the penis decreases, and subsequently the penis starts losing the erection and gets back to a flaccid state.