Regular marijuana use probably doesn't cause psychiatric problems, but it may make existing problems worse, a new study suggests.

Or maybe it is the other way around: People continue using the drug because of psychiatric difficulties.

Two doctors writing in the American Journal of Psychiatry are left with that puzzle after studying 100 regular marijuana users and 50 nonusers over a six- to seven-year period.

In that time, they found both the users and the nonusers had more diagnosable psychiatric disorders than they had at the start of the study, suggesting that the drug does not cause new psychiatric problems.

But those users who already were suffering from depression, alcohol abuse and antisocial personality at the beginning of the study seemed to have those problems more severely at the end.

Because all the subjects of the study were volunteers, the researchers write, they may have been predisposed to have mental problems, skewing the results.

"An additional possibility," write Drs. Ronald A. Weller and James A. Halikas, "is that marijuana use may have exacerbated preexisting psychiatric difficulties and made them more apparent."

What has yet to be determined, he writes, is whether such a connection would lead people to smoke more marijuana or less.