Degradation of cyclic GMP by an enzyme PDE5 lowers the cyclic GMP levels, causing outflow of blood from the chambers and loss of erection. Its major mode of action is inhibition of the enzyme PDE5. Inhibiting this enzyme allows the cyclic GMP to stay around longer, thus maintaining erection. It mainly enhances the normal physiologic erection. Elderly men and men with liver or kidney dysfunction have reduced ability to eliminate the drug.
Side effects are dose related.