Serotonin syndrome is a disorder resulting from excess stimulation of serotonin and is associated with drug interaction, single-drug therapy, and overdose. We report a case involving a 32-year-old man who developed sudden agitation, diaphoresis, subjective fever, tremor, and insomnia. These symptoms were related to doubling the dose of citalopram in combination antidepressant therapy. Discontinuation of the agent resulted in early notable clinical resolution after 1 week. This is a rare report of serotonin syndrome induced by citalopram polytherapy. Although serotonin syndrome is rare, clinicians need to recognize it early.