Smoking marijuana produces euphoria and a range of psychological and physical effects that can be unpredictable at times.

The stereotype of pot smokers as lackadaisical loafers is supported by new research: People who smoke marijuana regularly over long periods of time tend to produce less of a chemical in the brain that is linked to motivation, a new study finds.

Researchers in the United Kingdom scanned the brains of 19 regular marijuana users, and 19 nonusers of the same sex and age, using positron emission tomography (PET), which helps measure the distribution of chemicals throughout the brain.

They found that the long-term cannabis users tended to produce less dopamine, a "feel good" chemical in the brain that plays an important role in motivation and reward-driven behavior. [Trippy Tales: The History of 8 Hallucinogens]

Study participants who smoked marijuana regularly, and those who began using the drug at a younger age, had lower levels of dopamine in a part of the brain called the striatum, which could be why cannabis users appear to lack motivation.

However, "whether such a syndrome exists is controversial," said study lead author Michael Bloomfield, a researcher at the Institute of Clinical Sciences at Imperial College London.

The people in the study used cannabis quite heavily, they all began using the drug between ages 12 and 18, and they all had experienced symptoms of psychosis while under the influence, the researchers said. Some of these symptoms include experiencing strange sensations while on the drug, or having bizarre thoughts, such as thinking they were being threatened by an unknown force.

Because increased dopamine production has been linked with psychosis, the researchers expected to find higher levels of dopamine in the cannabis users, but instead, their findings suggested the opposite.

Previous studies looking at marijuana's effects on the brain have shown that chronic marijuana use may trigger inflammation in the brain, which could affect coordination and learning, and that cannabis users have a higher risk of schizophrenia.

But the new results suggest more research is needed to understand the potential links between chronic marijuana use and mental illnesses.

"It has been assumed that cannabis increases the risk of schizophrenia by inducing the same effects on the dopamine system that we see in schizophrenia, but this hasn't been studied in active cannabis users until now," Bloomfield said in a statement. The results tie in with previous addiction research showing that substance abusers have altered dopamine systems.

The findings could explain behaviors commonly seen in marijuana users, not only those who may suffer psychosis symptoms or dependence, although further study is needed to better understand the link, the researchers said.

They also said the brain changes are likely reversible — previous studies did not find differences in dopamine production between former marijuana users and people who were never regular users of marijuana.

The detailed results of the study were published online June 29 in the journal Biological Psychiatry.

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