In a prospective hospital-based cohort study, we addressed the question of severity and outcome of antidepressant poisonings in patients who attended the Loghman-Hakim Hospital Poison Center, the only national center in Tehran dedicated for detoxification. The aim of the study was to find out if tricyclic antidepressant (TCA) intoxications require more therapeutic efforts than selective serotonin reuptake inhibitor (SSRI) intoxications. The study was applied over a 7-week period (28 March-20 May 2006). From 3578 intoxications, 334 patients with antidepressant or lithium self-poisoning were identified (9.3% of all poisoning cases; 233 females, 101 males; median age 24 years, min 13, max 70). Compared to SSRI single-substance intoxications (n=17), TCA single-substance intoxications (n=73) were associated with: (1) a significantly lower level of consciousness (P=0.005); (2) a significantly higher admission frequency (80.8 vs. 35.3%; P<0.001); and (3) a higher intubation frequency (13.7 vs. 0%; P=ns). SSRI multiple-substance intoxications were associated with a significantly lower level of consciousness than SSRI single-substance intoxications (P=0.042), while there was no significant difference between TCA multiple- and single-substance intoxications. This study suggests that an overdose with SSRIs results in a more favourable clinical outcome than an overdose with TCAs.