TL neuro

November 12, 2010

Does it matter which kind of cannabis you smoke?

Filed under: Cannabis, Memory — mtaffe @ 11:56 am

In this paper, Morgan and colleagues examined the memory function of cannabis smokers in a repeated-measures design. Individuals were tested on several tests which assess different aspects of memory function either 24 hrs after last smoking cannabis or immediately after smoking cannabis. The key manipulation was to group the cannabis users by the cannabidiol content of the cannabis they smoked prior to the acute challenge test.

ResearchBlogging.orgThe study used a convenience sample of regular cannabis users who were recruited by word of mouth and the snowball technique. This latter amounts to getting each subject recruited to attempt to get their friends and associates to participate as additional subjects. The researchers ended up with a sample of 134 users (36 Female) with an average age of 20.6 years who smoked cannabis, on average, 13.8 days per month.

There were two key memory tests. In the Prose Recall task, the participants heard a short prose passage and were asked to recall it as well as possible both immediately after hearing it, then again after a delay during which the remaining assessments were conducted. The additional tasks included assessment of categorical and verbal fluency (generate as many words as possible within a given category within 60 seconds) and of source memory (recall of the gender of the speaker presenting each item in a list of study words).
One of the most interesting and novel twists to this study was that the authors asked the users, who were assessed in their own home or that of a friend, for samples of the cannabis they smoked on the acute-challenge test day. They then analyzed the content of delta9-tetrahydrocannabinol (THC) and cannabidiol in the materials. The analysis of the data then were conducted by grouping the individuals who had smoked cannabis that contained negligible amounts of cannabidiol and those that smoked cannabis with at least 0.75% cannabidiol content. This latter amounted to only 22 individuals so the authors compared these with the 22 individuals who smoked cannabis with the lowest amount of cannabidiol and excluded the rest of the sample.

The major finding of the paper was that the mean score for the delayed recall task was impared by acute cannabis smoking, but the effect was only in the low-cannabidiol content smokers. This depended on the finding of reliable interaction between the testing conditions and the type of category of cannabis that was used. The post-hoc test confirmed that the low-cannabidiol group scored worse than did the high-cannabidiol group on the day of testing while intoxicated but not on the 24 hr abstinent day.

I take issue with the authors’ statement in the discussion that “The high- and low-cannabidiol groups did not differ in performance when drug free and thus this finding cannot be attributable to any pre-existing group differences.” This overlooks the fact that the two groups did differ numerically (the low-cannabidiol group was worse) and the error bars were reasonably tight. In such a case it brings up a question of statistical power to detect differences and a strong caution not to assume that the data prove the null hypothesis- after all, we can only reject or fail to reject the null hypothesis of no-difference.

In a similar vein, the authors found a significant interaction between the groups (high/low cannabidiol in preferred cannabis material) and the type of testing day (intoxicated versus 24 hr abstinent) for the verbal fluency task. Interestingly, it appeared to be the case that the low-cannabidiol group performed less well in the 24 hr abstinent condition relative to their own performance when intoxicated as well as the high-cannabidiol group in the off-cannabis condition. It was also the case that the low-cannabidiol group scored more poorly on the Wechsler Test of Adult Reading.

All together there is some evidence in this paper that the individuals who had obtained high-cannabidiol content cannabis may have had higher baseline cognitive ability than the other group. This makes it very difficult to conclude that the observed difference on the prose recall task after smoking cannabis was specifically attributable to cannabidiol content.

It is an intriguing result, without doubt. I just think the authors are a bit too optimistic about the strength of their data. Still, this sort of result tends to raise the bar for future investigations. Naturalistic studies should make some attempt to understand the cannabidiol content in the typical material smoked by their sample groups. Laboratory studies should attempt to manipulate cannabidiol content.

Related Reading
Cannabidiol attenuates the appetitive effects of Delta 9-tetrahydrocannabinol in humans smoking their chosen cannabis.

Neurophysiological and subjective profile of marijuana with varying concentrations of cannabinoids.
References Cited
Morgan CJ, Schafer G, Freeman TP, & Curran HV (2010). Impact of cannabidiol on the acute memory and psychotomimetic effects of smoked cannabis: naturalistic study: naturalistic study [corrected]. The British journal of psychiatry : the journal of mental science, 197 (4), 285-90 PMID: 20884951


  1. […] This post was mentioned on Twitter by Drug Monkey, E. Phat Ma. E. Phat Ma said: Does cannabidiol in #marijuana attenuate the memory-impairing effects of delta9THC? […]

    Pingback by Tweets that mention Does it matter which kind of cannabis you smoke? | TL neuro -- — November 12, 2010 @ 1:22 pm

  2. […] disruption from smoking cannabis depended on the amount of cannabidiol (CBD) in the material in a prior post. The CBD appeared to be protective as it prevented a cannabis-induced impairment of delayed recall. […]

    Pingback by THC and Cannabidiol Content of Marijuana Seized in California « Scripps Center for Cannabis Addiction Neurobiology — September 8, 2011 @ 9:51 am

  3. […] and colleagues have shown (blog writeup) that smoking cannabidiol-enriched marijuana does not cause the deficits of immediate and delayed […]

    Pingback by Cannabidiol fails to attenuate THC-induced hypothermia | TL neuro — November 23, 2014 @ 11:24 am

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