Serotonin got famous as the joy chemical—the one that could lift depression and put us in a good mood. It’s also in the venom of many animals, from hornets to snakes, and in some cases it’s lethal.
The vast majority of the serotonin in your body is in your guts. There, it helps regulate your digestion. The small amount of serotonin in your brain helps regulate feeling like happiness and contentment. It keeps your appetite at an appropriate level and lets you know when it’s time to sleep and when it’s time to wake up.
Take some of that serotonin from out of your gut, your blood, your brain, purify it, and inject it just under your skin—and it will hurt like crazy. Serotonin is a component of hornet venom, of snake venom, of scorpion venom, and of sting ray venom. Serotonin helps regulate digestion by activating smooth muscle—like the muscle around the bladder, intestines, and uterus. These muscles, when triggered hard, can cramp up and hurt us. Smooth muscle also surrounds blood vessels, and these muscles are equally capable of constricting and cramping. That’s part of the pain we feel when we get stung by a hornet, or otherwise envenomated. In small animals, the serotonin can do more than hurt. Smooth muscle can constrict the blood vessels so much that they choke off the blood supply entirely and kill the animal. So don’t assume that serotonin will always make you happy.