Serotonin syndrome Serotonin syndrome, or otherwise known as serotonin toxicity, is a potentially life-threatening condition. The condition is associated with increased serotonergic activity in the central nervous system (CNS). In the central nervous system (CNS), serotonin modulates attention, thermoregulation and behavior. Serotonin ( in the PNS) and is involved in vasoconstriction, uterine contraction and bronchoconstriction. It is important for clinicians to understand that Serotonin syndrom