Serotonin, known as the “happiness” neurotransmitter, is a chemical found in the body responsible for feelings of well-being. But serotonin isn’t the only chemical that binds to the 13 serotonin receptors found on the surface of cells. Far from it. Many approved drugs also bind to serotonin receptors. And one of these receptors—called 5-HT2BR—has made drug developers very unhappy. That’s because some drugs that treat Parkinson’s disease, migraines, pituitary tumors, and obesity were designed to target other cellular receptors but also activate 5-HT2BR, leading to life-threatening valvular heart disease. As a result, many of these drugs have been pulled from the market.