Suicide Seasonality : Theoretical and Clinical Implications

Background: Although suicide seasonality has been well-documented, surprisingly little is known about its underlying mechanisms. Methods: In this thesis, data from three Swedish registers (Cause of Death Register, National Patient Register, Prescribed Drugs Register) and data from the Swedish Meteorological and Hydrological Institute were used. In Study I, the amplitude of suicide seasonality was estimated in completed suicides in 1992-2003 in individuals with different antidepressant medications or without antidepressants. In Study II, monthly suicide and sunshine data from 1992-2003 were used to examine the association between suicide and sunshine in groups with and without antidepressants. In Study III, the relationship between season of initiation of antidepressant treatment and the risk of suicidal behavior was explored in patients with a new treatment episode with antidepressant medication. In Study IV, the complex association between sunshine, temperature and suicidal behavior was investigated in patients with a new treatment episode with an antidepressant in two exposure windows (1-4 and 5-8 weeks) before the event. Findings: Study I: Higher suicide seasonality was found in individuals treated with selective serotonin reuptake inhibitors (SSRIs) compared with those given a different antidepressant treatment or those without any antidepressant treatment. Study II: In individuals treated with SSRIs, there was a positive association between sunshine and suicide, with the association stronger in men treated with SSRIs compared with men treated with other antidepressants. An effect modification by age was observed. Study III: The elderly (65+) had a higher risk of suicide when initiating antidepressant treatment in summer and a higher risk of suicide attempt when starting antidepressant therapy in spring and summer. Younger patients (0-24) demonstrated a higher risk of suicide attempt when treatment was initiated in autumn. Study IV: In the elderly (65+), a harmful association was observed between the risk of suicide attempt and the average daily temperature during the four weeks before the suicide attempt, as well as with average daily sunshine during both exposure windows (1-4 and 5-8 weeks) before the suicide attempt. Significance: Our results provide epidemiological support for the role of the serotonergic system in seasonality of suicide in which both medication and climate may be involved