Serotonin syndrome (SS) is an underdiagnosed drug-induced clinical syndrome resulting from the excess intrasynaptic concentration of serotonin. Very limited information is available about chronic SS.

To evaluate the epidemiological, clinical, and other aspects of the insidious onset SS.

We retrospectively evaluated 14 consecutive adult patients (> 18 years) who had complaints for more than 6 wk at the time of consultation and met the Hunter criteria for SS.

The mean age was 41.1 years (range: 21-61 years), with a male preponderance (64%). Although tremors were observed in all patients, this was a presenting complaint in only 43% of patients. Generalized body pain, insomnia, and restlessness were common presenting features (50% each). Other common clinical features were stiffness of the limbs (43%), diaphoresis (43%), gait disturbances (36%), bowel disturbances (36%), dizziness (29%), sexual dysfunctions (21%), incoordination (14%), and fatigue (14%) The mean duration of symptoms before the diagnosis of SS was 13.5 ± 5.8 wk (range: 6-24 wk). Amitriptyline was the most common drug (n = 6, 43%), followed by tramadol (n = 5, 36%) and sodium valproate (n = 5, 36%). All patients received cyproheptadine, a 5- hydroxytryptamine2A antagonist, as treatment and noted an excellent response over the course of 4-14 d.

This study represents the largest study on chronic SS. We suggest that patients receiving serotonergic drugs should be physically examined for the presence of SS upon the development of new symptoms.

• Serotonin syndrome
• Serotonin toxicity
• Cyproheptadine
• Serotonergic agents
• Serotonin
• Clonus