Viagra improves a man's response to sexual stimulation. To understand how, you need to understand how an erection happens in the first place.

Erections are all about increasing blood flow to the penis. Without sexual stimulation, the body limits blood flow into the penis. This keeps the penis flaccid (soft or not erect).

When a man is sexually aroused, the arteries in the penis relax and widen, allowing more blood to flow into the penis. As the penis expands and hardens, veins that normally carry blood away from the penis become compressed. This restricts how much blood can flow out of the penis. With more blood flowing in and less flowing out, the penis enlarges, resulting in an erection.

If the nerves or blood vessels associated with this process aren't working properly, a man may not be able to get an erection.

Viagra increases blood flow to the penis, so that when a man is sexually aroused, he can have an erection. When the sexual encounter is over, his erection goes away.

If a man takes Viagra and then circumstances change and he isn't sexually stimulated, he won't get an erection. You won't get an erection just by taking the pill.

Viagra is not a hormone. Viagra is not an aphrodisiac. It is a prescription medicine that can improve the erectile function of many men with erection problems.