Spiders of the genus Phoneutria, known as Brazilian wandering spiders, are some of the deadliest in the world. Just 0.006 mg of their venom—about the same mass as two grains of sand—can kill a mouse. A bite will also cause a grown man intense pain, blurred vision, vomiting, a spike in blood pressure and heart rate, tremors, and ... a noticeable bulge in the pants. You read that right. One of the consequences of a Phoneutria bite (in men) is an erection. This is not a good thing: It’s painful, prolonged, and can cause permanent damage to the penis. But there is an upside. The venom ingredient causing the situation—a peptide called Tx2-6—can be isolated, and researchers have shown it can improve erectile function in mice. They hope the chemical can eventually be developed into an erectile dysfunction drug.

This story originally appeared in the September 2014 issue of mental_floss magazine. Subscribe to our print edition here, and our iPad edition here.