The study of 86,626 nurses in 1980-90 found 11 suicides among those who drank two to three cups of caffeinated coffee a day compared to 21 cases among colleagues who said they almost never drank coffee.
"Coffee drinkers seem to do everything that seems to put them at risk for depression and suicide but they are highly protected," said the study author, Ichiro Kawachi, of Harvard Medical School and Brigham and Women's Hospital, Boston. He said many coffee drinkers lead stressful lives and smoke and drink heavily. The study did not examine whether respondents were told not to drink coffee, nor question the effect of caffeine on people who attempt suicide. Dr Kawachi said the issues merit further study.
A 1990 study found 100mg of caffeine a day could produce increased feelings of well-being, energy and motivation to work. A 5oz cup contains 40 to 180mg of caffeine, the US Food and Drug Administration says.
John Greden, an expert in depression at the University of Michigan, said researchers should have examined the effect of anti-depressants and blood-pressure medication, which tends to be a depressant. They also should have looked at how many suicide victims tried to stop smoking, which also can trigger depression.
"The findings could have nothing to do with caffeine," Dr Greden said.