Importance of the work and objectives: Lithium occurs naturally in drinking water and may have a positive effect on mental health and suicide. In clinical practice, lithium in high therapeutic doses is used as a mood-stabilizer in the treatment of affective disorders. Previous studies performed at an ecological level have found an association between lithium in drinking water and risk of suicide. The present study is the first to investigate this association at an individual level considering long-term exposure.

Methodologies: The study population consisted of all 3,724,588 Danish adults (≥20 years) of which 15,370 committed suicide from 1990-2012. Information on suicides was obtained from the nationwide Danish Register of Causes of Death. Data on lithium concentrations were obtained through a nationwide drinking water campaign from 2013 including 151 measurements from waterworks supplying approximately 42% of all residents in Denmark. Spatial statistics were applied to investigate geographical patterns in lithium levels and to compute an accumulated lithium exposure for each individual. Poisson regression analyses were used to investigate the association between accumulated lithium exposure and suicide rate.

Main results and conclusions: Significant regional clustering in drinking water lithium levels were found with high levels in Eastern and low levels in Western Denmark. A significant dose-response trend of decreasing suicide rates with increasing lithium exposure was found even after adjustment for socioeconomic factors. This study supports the growing evidence of naturally occurring lithium being protective against suicide, which, if further supported, may have implications for future public health strategies on suicide prevention.