Viagra. Levitra. Cialis. Stendra. For millions of men with erectile dysfunction, these drugs are the action heroes of the bedroom, breaking down the barriers that keep them from a normal sex life. Here’s how they work.

A normal erection depends on a complex dance of relaxation and blood flow. When the penis is flaccid, signals from one set of nerves keep muscles inside penile arteries squeezed tight and blood flow at a relative trickle. Arousal damps down that signal, and simultaneously sparks a second set of involuntary nerves to get the party going. The signal from those nerves tickle the inner lining of the penile arteries, starting a chain reaction inside their cells that builds a molecule called cGMP. cGMP relaxes arterial smooth muscle inside the penis, opening the floodgates and filling the organ with blood.

But erections don’t last forever — an enzyme that breaks down cGMP eventually returns things to status quo. As cGMP levels fall, penile arteries contract and blood flow through the erectile tissue returns to its normal trickle.

When men develop cardiovascular diseases, the inner lining of the penile arteries can get damaged. Damaged lining can’t make enough cGMP, and without that trigger the arteries won’t relax enough to get the blood flowing. Without enough blood, the penis stays too soft for penetration.

Here’s where the pills come in. The brand name doesn’t matter: all of these drugs grab onto the enzyme that destroys cGMP and hold it hostage. Protecting cGMP from harm lets enough of it build up in penile arteries to turn on some ‘relaxation’ action and get a move on penile expansion.

Strictly speaking, popping one of these pills won’t instantly flip a penis to the on position — you still need some form of erotic stimulation to get things going. Instead, the drugs let some guys do a lot more with the little bit of signal they still make. That’s why the drugs aren’t appropriate for everyone: they can’t do much for people whose erectile dysfunction doesn’t stem from a signal shortage, and they won’t “enhance” erections in guys with normal function. (In fact, turning down the “off” switch in young guys can sometimes lead to erections that last so long they damage the penis, but that’s a subject for another day.)