Dr. Michel Lucas and colleagues1 leveraged three large studies of US men and women—the Health Professionals Follow-up Study (1988–2008; n=43,599 men), the Nurses’ Health Study (1992–2008; n=73,820 women), and the NHS II (1993–2007; n=91,005 women) in which consumption of caffeine, coffee, and decaffeinated coffee was assessed every 4 years by validated food-frequency questionnaires. (Although the researchers looked at caffeine consumption from other sources such as tea, soft drinks, and chocolate, they found the major caffeine source was coffee.) In total, there were 277 deaths as a result of suicide.
In examining the pooled multivariate relative risk, Lucas et al. found that drinking caffeinated coffee actually decreased the risk of suicide. Specifically, drinking at least two to three cups (8 oz) of caffeinated coffee per day seemed to reduce the risk of suicide by about 50% as compared to those participants who consumed 1 or less cup of coffee per day. Lucas and colleagues found only small increases in benefits for drinking more than 3 cups of caffeinated coffee per day.
“Unlike previous investigations, we were able to assess association of consumption of caffeinated and non-caffeinated beverages, and we identify caffeine as the most likely candidate of any putative protective effect of coffee,” Lucas said in a press statement.2
Previous research has suggested that caffeine boosts such neurotransmitters as serotonin, dopamine, and noradrenaline in the brain, which have mild antidepressant effects. This may be the reason behind the apparent reduced suicide risk. Nonetheless, there are negative effects associated with caffeine, so the researchers cautioned patients and clinicians about large amounts of caffeine intake.
1. Lucas M, O'Reilly EJ, Pan A, Mirzaei F, Willett WC, Okereke OI, Ascherio A. Coffee, caffeine, and risk of completed suicide: Results from three prospective cohorts of American adults. World J Biol Psychiatry. 2013 Jul 2 [Epub].
2. Dwyer M. Drinking coffee may reduce risk of suicide in adults. Harvard University Press Release. July 24, 2013. Available at: http://www.hsph.harvard.edu/news/features/drinking-coffee-may-reduce-risk-of-suicide-in-adults/. Accessed September 13, 2013.
First of all, the coffee consumption didn't prevent the suicides -- it was associated with lower risk of suicide. Potential confounds include being employed -- you can't take a coffee break if you don't have a job, and you are more likely to die by suicide if you are unemployed.