The chemical serotonin is most commonly known as a mood regulator—in the human brain, it is believed to help promote feelings of happiness and is often studied in conjunction with mood disorders like depression. However, serotonin has a range of different functions in the human body. In fact, it’s actually primarily found working in the gastrointestinal system, where it regulates digestion by controlling smooth muscle contraction. 

According to io9, serotonin is also a component of many animal venoms. Scorpions, stingrays, snakes, and hornets all use serotonin when stunning or stinging their prey. 

So, how is a chemical that’s famously associated with a range of healthy biological functions also used as a weapon? 

It turns out that when injected into the skin, serotonin causes intense pain. That’s because it’s essentially doing the same job it does in your intestine—causing smooth muscle to contract—but in the wrong place. When the smooth muscle around blood vessels in your skin contracts (a process called vasoconstriction), it can cause severe local pain, according to Encyclopedia of Life

So, while serotonin is most famously known as the "joy chemical,” it’s also a highly effective weapon. While a serotonin injection doesn't seem to be lethal for humans (though it can be for smaller animals), it's a component in some of the most dangerous venoms in existence. While that's a bit scary, it's also a fascinating illustration of serotonin's versatility.

[h/t io9]