Claims cannabis 'rewires the brain' misleading

“Cannabis use ‘shrinks and rewires’ the brain,” reports The Daily Telegraph, with much of the media reporting similar “brain rewiring” headlines.

The headlines are based on a study that compared the brain structure and connections of cannabis users with those of non-users.

The researchers identified several differences between cannabis users and non-users in a region of the brain called the orbitofrontal cortex.

This is part of the reward network, and is enriched with cannabinoid 1 receptors. These bind THC, the active ingredient in cannabis.

Some of the differences seen by the researchers were associated with how long people had used cannabis or the age they started using the drug.

However, although brain differences were found, it is not clear they were caused by cannabis use. It is possible that brain differences mean it is more likely that certain people use cannabis.

Where did the story come from?

The study was carried out by researchers from the University of Texas, The Mind Research Network and the University of New Mexico.

It was funded by the US National Institute on Drug Abuse.

The study was published in the peer-reviewed medical journal PNAS. This article was open access, so is free to read online.

The media generally reported the results of this study along the lines of The Guardian’s headline: “Smoking cannabis every day ‘shrinks brain but increases its connectivity’.” But these headlines are misleading.

This study did find differences between the brains of cannabis users and non-users, but because it was only a snapshot in time, we can’t tell if the brain differences were caused by cannabis.

It is possible that brain differences mean it is more likely that certain people use cannabis. These could be pre-existing differences in the parts of the brain associated with feelings of reward, and people with this brain structure are more likely to try or persist in using cannabis.

What kind of research was this?

This was a cross-sectional study that compared the brain structure and connections of people who used cannabis with the brain structure of non-users to see if there were any differences.

Although this type of study can identify differences in brain structure and connections between cannabis users and non-users, it cannot show that the differences were caused by cannabis use: people with different brain structures may be more likely to use cannabis, for example.

What did the research involve?

The researchers used magnetic resonance imaging (MRI) scans to look at the brains of 48 cannabis users, who were using cannabis at least four times a week over the previous six months, and 62 non-users.

The cannabis users varied in age, and the non-users were chosen because they were the same sex and age as the users.

The researchers also used the marijuana problem survey to assess the negative psychological (such as feeling bad about marijuana use), social (such as family problems), occupational (such as missing work), and legal consequences of marijuana use over the previous 90 days.

What were the basic results?

The researchers identified several differences between the brains of cannabis users and non-users.

These differences were in a region of the brain called the orbitofrontal cortex. This is part of the reward network of the brain, and is enriched with cannabinoid 1 receptors that bind THC (the “active” ingredient in cannabis).

The researchers found the orbitofrontal cortex was smaller in cannabis users, but there was more connectivity.

Some of the brain differences were correlated with behaviour related to cannabis. Some brain differences varied with duration of use, and some of the differences were associated with the age a person had started using cannabis.

How did the researchers interpret the results?

The researchers say that their findings “suggest that chronic marijuana use is associated with complex neuroadaptive processes, and that onset and duration of use have unique effects on these processes”.

Conclusion

Links To The Headlines

Smoking cannabis every day ‘shrinks brain but increases its connectivity’. The Guardian, 10 November 2014

Cannabis use ‘shrinks and rewires’ the brain. The Daily Telegraph, 10 November 2014

Smoking cannabis every day ‘warps your brain and shrinks grey matter’, scientists warn. Daily Mail, 10 November 2014

Links To Science

Filbey FM, et al. Long-term effects of marijuana use on the brain. PNAS. Published 10 November 2014