Other neurotransmitter systems such as dopamine, serotonin, γ-aminobutyric acid (GABA), norepinephrine (noradrenaline), and glutamate are also thought to contribute to critical illnessassociated CD. Dopaminergic hyperfunction is thought to underlie the cognitive symptoms of schizophrenia, and dopamine administration itself may be a risk factor for delirium [29] . Serotonin syndrome, a consequence of excess serotonergic agonism, can be seen not only with selective serotonin reuptake inhibitors but also with a variety of drugs and drug combinations [30] . Even a single therapeutic dose of an selective serotonin reuptake inhibitor can cause the syndrome, which manifests as mental status changes, autonomic hyperactivity, and neuromuscular abnormalities.