All PDE-5 inhibitors work in the same way. These include Viagra, Sildenafil, Cialis, Tadalafil, Levitra, Vardenafil and Spedra.
When you are aroused your tissues release a chemical called nitric oxide (NO), this, in turn, causes the release of a second chemical called cyclic guanosine monophosphate (cGMP). cGMP causes the muscles at the sides of your penis to relax allowing blood to rush in. The veins that drain the blood from your penis also get compressed trapping the increased blood in your penis resulting in an erection.
An enzyme called phosphodiesterase-5 (PDE5) breaks down the cGMP and your penis returning it to its original state. PDE5 inhibitors such as Viagra/Sildenafil, work by blocking the action of PDE5, so your levels of cGMP remain high, the muscles at the side of your penis relax, more blood rushes in, and you can achieve a firmer and longer lasting erection. It is important to remember that PDE5 inhibitors only exert their action when you are sexually aroused, it does not increase your sexual desire, and it will not resolve any underlying physical causes of your ED.