At least 38 men who have taken Viagra have gone blind, and the drug’s manufacturer said last Thursday that it would consider adding a warning to the label. Drug companies already warn consumers that Viagra and competitors like Levitra and Cialis have several other side effects, like blurred vision, headaches, indigestion, and painful erections that last four hours or more. What do you do with an erection that won’t quit?

Call your doctor—and be very, very afraid. Though the makers of erectile dysfunction drugs list prolonged, unwanted erections as a potential side effect, urologists have their doubts. Priapism, a rare condition defined by prolonged erections in the absence of sexual arousal, is associated with certain blood diseases, hypertension, and recreational drug use. No matter what its cause, priapism can be dangerous if left untreated. A man who has a painful erection for more than 12 hours is at high risk for permanent damage.

Under normal conditions, a man gets an erection when his nervous system triggers the dilation of the arteries that feed into his corpora cavernosa, spongy tissues that girdle the urethra along the length of the penis. These fill with blood and harden, as do both the spongy tissue that encapsulates the urethra and the head of the penis, called the glans. This constricts the veins leaving the penis, temporarily maintaining an erection.

So, what causes an erection that won’t go away? There are two types of priapism, non-ischemic and ischemic. Non-ischemic priapism usually results from an injury to the penis or perineum. If an artery that feeds into the corpora cavernosa ruptures, blood might spill into the erectile tissues. While blood can now flow both in and out, the outflow is not sufficient to shrink the erection. As a result, a man with non-ischemic priapism will have a semirigid penis for hours on end. Because the circulation of blood has not been impeded, non-ischemic priapism doesn’t hurt and often clears up on its own without causing long-term damage. Treatment usually consists of little more than an ice pack.

In the case of ischemic priapism, blood flows into the penis but doesn’t flow out. The loss of circulation deprives the corpora cavernosa of oxygen and causes a painful, rigid erection. If left untreated, the loss of oxygen can damage erectile tissues, cause the formation of scar tissue, and lead to a permanent loss of function.

The first treatment for ischemic priapism is “therapeutic aspiration.” The urologist sticks a needle into the side of the penis and draws blood directly from the cavernosa. The same needle is sometimes used to inject saline solution into the penis, which can help flush out the remaining blood. Aspiration and irrigation work around 25 percent to 30 percent of the time. Doctors can also inject drugs that constrict the arteries and cut off penile blood flow. Injections are given every few minutes for about an hour.

In rare cases, an intractable erection will resist both aspiration and drug injections. In this case, the urologist performs a surgical shunt. The simplest type of shunt involves cutting a small hole in the head of the penis, then opening oval windows in the tips of the cavernosa. When the surface incision is closed, the blood trapped in the erectile tissue drains out through the unaffected penile veins. A more involved shunt procedure involves cutting a nearby but unrelated vein and attaching one end of it to the base of the penis.

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Explainer thanks Drs. Ira Sharlip and Drogo Montague.