Whether it is the caffeine or some other chemical in coffee, the dark brew does seem to have at least a mild antidepressant effect. A number of studies have come to this conclusion by comparing the suicide rate of coffee drinkers with the rate of those who abstain from coffee.

The most recent study showing a suicide-preventive effect from coffee drinking was especially noteworthy, as it was large-scale and adjusted for confounding factors. Published in the Archives of Internal Medicine in 1996, the study followed more than 86,000 registered nurses in the U.S., aged 34 to 59, for ten years. The researchers found that women who drank two to three cups of coffee per day were significantly less likely to commit suicide compared to those that did not drink any coffee. The researchers were aware that many factors might also affect suicide rates, such as alcohol intake, marital status, stress level, cigarette smoking, and overall health. The authors noted that the coffee drinkers were more likely to also smoke and drink, and that coffee drinkers reported more stress. Even so, when the researchers statistically corrected for all these and other potential variables, the coffee drinkers still were much less likely to commit suicide.

A similarly large study, covering more than 130,000 nurses over eight years, also reported reduced suicide levels among coffee drinkers.

The caffeine in coffee may have mood-elevating actions through effects on neurotransmitters such as dopamine and acetylcholine. It is also possible that coffee drinking has social effects, such as increasing personal contacts or sociability, that might reduce thoughts of suicide.

Reference: Kawachi, I., et al., "A prospective study of coffee drinking and suicide in women," Arch Intern Med (1996), 156(5):521-25