1. Addition of a serotonergic agent to an already established treatment (or increase in dosage) and manifestation of at least 4 major symptoms or 3 major symptoms plus 2 minor ones
Mental (cognitive and behavioral) symptoms
Major symptoms: confusion, elevated mood, coma or semicoma
Minor symptoms: agitation and nervousness, insomnia
Autonomic symptoms
Major symptoms: fever, hyperhidrosis
Minor symptoms: tachycardia, tachypnea and dyspnea, diarrhea, low or high blood pressure        
Neurological symptoms
Major symptoms: myoclonus, tremors, chills, rigidity, hyperreflexia
Minor symptoms: impaired co-ordination, mydriasis, akathisia
2. These symptoms must not correspond to a psychiatric disorder, or its aggravation, that occurred before the patient took the serotonergic agent.
3. Infectious, metabolic, endocrine or toxic causes must be excluded.
4. A neuroleptic treatment must not have been introduced, nor its dose increased, before the symptoms appeared.

Table 1: Radomski’s Revised Diagnostic Criteria for Serotonin Syndrome