ED drugs like Viagra are vasodialators, they increase blood flow. This is handy for men who can't seem to get enough blood to their erection to keep it erect.
For women it also increases the amount of blood flow to the genitalia during sexual arousal thus heightening their sensitivity to touch. This often results in increased sexual arousal and response to stimulation.
Pro tip: ED drugs (Viagra, Cialis, ect) are vasodialators. In particular they are arginase inhibitors. Arginase is an enzyme which plays a role in the body's processing of arginine, the main precursor to nitric oxide which is the primary known controller of human vascular dialation.
So basically ED drugs increase levels of nitric oxide which leads to vasodilation by increasing the amount of available arginine. They do this by lowering levels of the enzyme which breaks down arginine for processes other than nitric oxide synthesis.
So you can pay for expensive prescription drugs to boost levels I arginine or you could just take extra arginine in the form of a supplement. To supercharge the process you can add citrulline to the mix, which is a precursor to arginine in the body.
Trust me, 5g arginine HCl and 3g citrulline maltate will have you feelin' like Ron Jeremy