Serotonin syndrome, also known as serotonin poisoning, serotonin sickness, and many other names, is a medical condition that can happen if there is too much serotonin in your brain or other parts of your nervous system. Serotonin syndrome can be mild (doesn't make you very sick), but in rare situations, it can kill you. Serotonin syndrome can happen if you use some drugs that can increase the amount of serotonin in your nervous system. Some examples of drugs that can make you have serotonin syndrome are antidepressants, opioids, CNS stimulants, 5-HT agonists (a drug that changes the receptors of serotonin), psychedelics, herbs, and many others.