Smoking weed could blunt brain's motivation system

A new study has found that long-term cannabis users tend to produce less dopamine, a chemical in the brain linked to motivation.

Researchers found that dopamine levels in a part of the brain called the striatum were lower in people who smoke more cannabis and those who began taking the drug at a younger age.

They suggest this finding could explain why some cannabis users appear to lack motivation to work or pursue their normal interests.

The researchers at Imperial College London, UCL and King's College London used PET brain imaging to look at dopamine production in the striatum of 19 regular cannabis users and 19 non-users of matching age and sex.

The cannabis users in the study had all experienced psychotic-like symptoms while smoking the drug, such as experiencing strange sensations or having bizarre thoughts like feeling as though they are being threatened by an unknown force.

The researchers expected that dopamine production might be higher in this group, since increased dopamine production has been linked with psychosis. Instead, they found the opposite effect.

The cannabis users in the study had their first experience with the drug between the ages of 12 and 18.

There was a trend for lower dopamine levels in those who started earlier, and also in those who smoke more cannabis.

The researchers led by Dr Michael Bloomfield, from the Institute of Clinical Sciences at Imperial said that these findings suggest that cannabis use may be the cause of the difference in dopamine levels.

The study is published in the journal Biological Psychiatry.