Cannabis Sense

A study finding brain changes in casual users of marijuana got a lot of press last week.

There were people seizing upon it as proof of marijuana’s danger and other attacking or debunking the study. Those who attacked the study seemed to react to the inferences people were drawing from the study’s findings, rather than dealing with the actual findings.

I held back because there seemed to be much more heat than light.

Now, finally, we hear from a dispassionate voice of reason that examines the actual findings. The U.K. National Health Service provided this analysis of the findings:

This study found differences between young recreational cannabis users and non-users in the volume and structure of the nucleus accumbens and amygdala, which have a role in the brain’s reward system, pleasure response, emotion and decision making.

However, as this was only a cross sectional study taking one-off brain scans of cannabis users and non-users, it cannot prove that cannabis use was the cause of any of the differences seen. It is not known whether cannabis use could have caused these changes in regular users.

Or conversely whether the cannabis users in this study had this brain structure to start with, and that this may have made them more likely to become regular users of cannabis.

Also, this is a small study comparing the brain structure of only 20 users and 20 non-users. With such a small sample of people, it is possible that any differences in brain structure could have been due to chance. These changes may not have been evident had a larger number of people been examined.

Examination of different samples of people, and in different age groups, may have given different results.

Similarly, examining the extent of brain structural change was related to factors such as age at first use, and frequency or duration of use, are less reliable when based on such a small sample of people.

Confirmation of these tentative findings through study of other groups of cannabis users is now needed.

It would also be of value to see whether the structural differences observed actually correlated with any demonstrable differences in thought processes and decision making behaviour.

It’s a shame that this has, somehow, turned into a front in the culture wars.

Mark Kleiman  questions the motives criticized the unjustified implications* (but not the data) of the researchers:

Overall then, if you were that neuroscientist, you’d write a paper saying “We studied cannabis users and non-users and found the following brain differences. Here’s the next study we plan to do, addressing the questions of causation and possible impact.”

That’s assuming that your goal was informing your readers about the content of your findings. If instead you wanted to score points in the culture wars, push your political agenda, and perhaps please your sponsors at the National Institute on Drug Abuse and the Office of National Drug Control policy, you’d behave differently.

He also challenges the users be framed as “casual users”:

Pretending that the findings to “casual” cannabis user would require that you gloss over how extreme your sample was: an average age of onset of just over 15 (very young exposure is known to be correlated with higher risks) and cannabis use of a minimum of a joint a week and an average of 11 joints a week. (The median cannabis user consumes once a month; once a week – the minimum in this study – puts someone in the top quartile, while 11 joints a week would put someone in the top 15%.) Instead, you’d describe your findings as applying to “recreational” or “light-to-moderate” cannabis use.

The then ends with a point that will disappoint some people who’d been cheering him on:

It’s entirely possible, though not yet demonstrated, that chronic heavy cannabis use causes undesirable changes in brain structure and function. Even if it doesn’t, spending a good chunk of your waking hours zonked seems to me like a bad idea no matter what the zonking agent is, and that’s true in spades for adolescents, who may be unable to make up missed opportunities for both formal and social learning.

[* updated to reflect Kleiman’s comment below]