Pot smokers wish weed caused “reefer madness," because if it did, they'd be running around en masse, making things happen.

But a new study shows that smoking marijuana suppresses one's ability to synthesize dopamine — the neurotransmitter associated with motivation. Alas, the couch is where it's at.

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Researchers at three London universities used PET scans to analyze the dopamine levels present in the brains of 19 people who smoked cannabis regularly and had been doing so since they were teenagers. These people were also experiencing psychotic symptoms while under the influence. Since pot is known to affect dopamine activity and abnormal dopamine level is associated with schizophrenia, the research wanted to see if there might be a connection between smoking weed and having psychotic episodes.

The control group said they had used cannabis fewer than 10 times in their life, and at the time of the study, hadn't used it at all in the last three month.

Contrary to their hypothesis that habitual pot use increased dopamine and might contribute to psychosis, the researchers found that people who smoked pot weekly had lower dopamine levels than those in the control group. The lack of dopamine could explain the lack of drive that characterizes those who partake.

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Other studies have shown that drugs such as cocaine, methamphetamine, alcohol and nicotine stimulate the brain's dopamine circuits, giving folks a rewarding, feel-good buzz.

The London researchers report their findings online in Biological Psychiatry.