TUESDAY, July 2 (HealthDay News) -- Can chronic pot smoking really turn people into slackers?

According to a small brain-scan study, long-term marijuana use may indeed reduce people's motivation levels.

The findings could explain why some marijuana users lack the drive to work or pursue their previous interests, said the researchers at Imperial College London.

The researchers used PET brain imaging to assess dopamine production in the brains of 19 regular marijuana users and 19 non-users. Dopamine is a chemical linked to motivation. The brain scans showed that the regular marijuana users tended to produce less dopamine.

The marijuana users in the study had their first experience with the drug between the ages of 12 and 18. Those who started using the drug at an earlier age tended to have lower dopamine levels, as did those who were heavier users.

The researchers said these findings -- published recently in the journal Biological Psychiatry-- may be the cause of the differences in dopamine levels.

The lowest dopamine levels were seen in people who met diagnostic criteria for marijuana abuse or dependence. This suggests that measuring dopamine levels could provide a marker of a person's level of addiction to marijuana, the study authors said.

"[The findings] tie in with previous research on addiction, which has found that substance abusers -- people who are dependent on cocaine or amphetamine, for example -- have altered dopamine systems," study leader Dr. Michael Bloomfield said in an Imperial College London news release.

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