According to the report, adults who were not depressed when the study began, but who abused marijuana, were about four times more likely to report symptoms of depression 15 years later, compared with their non-smoking peers.

These adults were especially likely to have had suicidal thoughts and report a lack of interest in things that once held their interest, Dr. Gregory B. Bovasso reports in the December issue of the American Journal of Psychiatry. Pot smokers were four times more likely than their non-smoking peers to have suicidal thoughts, and white women were found to be at particular risk.

In the study, marijuana abuse was defined by various signs of problem pot use, such as impaired work performance or using the drug on the job.

Individuals who used other drugs such as amphetamines and opioids were about 8 to 10 times more likely to be abusing pot 15 years later. However, those who were depressed when the study began were no more likely to abuse marijuana later on, according to the report, which followed nearly 850 adults who were not depressed and more than 1800 who did not report marijuana abuse at the studys start.

In an interview with Reuters Health, Dr. Bovasso suggested that future studies investigate how excessive pot-smoking leads to a higher risk of depression, examine why adults abuse marijuana, and establish how much pot is enough to increase risk of depression.

Treatments or other interventions that prevent the abuse of cannabis from occurring in the first place are important,» Dr. Bovasso said. «On a general policy level, marijuana…may not be as harmless as many believe.»