BASICALLY, AN ERECTION is blood gathering in the sponge-like tissue in the penis, making it rigid. Drugs like Viagra dilate blood vessels at an artificially high level so the penis can hold more blood. Post-sex, if the blood is trapped and can’t drain out, it causes what’s called priapism.

That’s dangerous because, when an erection is prolonged, the penis may not get enough new oxygen, and that can cause injury even after just two hours, and critical tissue damage can start after four.

That said, the incidence of priapism in young men taking Viagra and the like is very low. We see it a lot more in men who try more potent erection boosters, like injecting medication directly into their penis, which can cause impotence. But guys who take antidepressants or anxiety meds or have certain genetic conditions (like sickle cell anemia) may be at higher risk.

If you still have a hard-on one to two hours after sex, don’t panic—it may naturally dissipate. If not, put an ice pack on it for 10 to 15 minutes, and if it doesn’t go down, go to the ER, where doctors will try to drain it with a needle/ syringe, meds, or even surgery.

Priapism is rare, but one stupid mistake, and you can lose your penis.

Benjamin Choi, M.D., is a clinical assistant professor in urology at Weill Medical Center of Cornell University, and an attending urologist at New York-Presbyterian/Weill Cornell Medical Center.