25

In their preliminary guideline, Authorizing Dried Cannabis for Chronic Pain or Anxiety, the College of Family Physicians of Canada (CFPC) has made a recommendation that dried cannabis not be prescribed to patients under the age of 25 (Recommendation 4.a). In Canada, patients under the age of 25 are regularly given psychoactive medication with uncertain long term cffects (amphetamines, atypical antipsychotics), often off label, just by virtue of their own consent and a shared decision making process with their prescriber. What is it about dried cannabis that makes it too dangerous to be prescribed to this 21 year old Canadian with hereditary multiple exostoses? The CFPC has given this a Level 2 recommendation, (based on well conducted observational trials), however they have reached a different conclusion than the regulatory authorities in Colorado and Washington, which have set 21 as the lower limit for the use of medical cannabis. How robust is the evidence to support the CFPC guideline recommendation that dried cannabis not be prescribed to those under 25?

Under the discussion and supporting evidence section we find this as the justification for this recommendation, with 6 references:

Patients under the age of 25 (Level II)
Youth who smoke cannabis are at greater risk than older adults for cannabis-related psychosocial harms,including suicidal ideation, illicit drug use, cannabis use disorder, and long-term cognitive impairment.

So let’s look at those references and see if they can shed light on why CFPC believes that 25 is the right number, rather than 21. Or 18?

The first reference is Meier, et al, 2012, often referred to as the Dunedin study. This study showed persistent cognitive impairment, and a possible decrease in IQ of 6 points. The accompanying editorial points out that these changes were only seen in certain circumstances:

Both adolescent onset and almost 2 decades of persistent cannabis use may be needed to obtain the magnitude and pervasiveness of long-term neuropsychological deficits reported by Meier et al

Many people who quote this study would also mention this followup article that appeared in the same journal later that year which suggests that the changes found in the Dunedin cohort were more likely due to socioeconomic factors rather than cannabis. In any event, this study is a good reason to be concerned about adolescent usage, a strong argument in favor of regulation and control of the marketplace.


The second study used as a reference (Fergusson, et al) also examined a New Zealand cohort and found that there was increased risk of crime, suicidal behaviour and other illicit drug use in a population of examined between the ages of 14–21, with the effects being much more pronounced in the younger population.

The findings reinforce public health concerns about minimizing the use of cannabis among school-aged populations.

The third reference is about the development of dependence symptoms in young cannabis users (aged 14–24). They found that 65% of cannabis users showed no signs of dependence. They did find at least one criteria for dependence in the rest and concluded:

Regular cannabis use in adolescence is associated with the development of a dependence syndrome.

The fourth reference is: “Early onset cannabis use and progression to other drug use in a sample of Dutch twins”. This is an attempt to support the “gateway effect” hypothesis,which states that cannabis leads to harder drugs. I think that there is poor evidence for this hypothesis, and there are specific criticisms that can be directed at this particular study. This would once again support the notion of being concerned about youth access to cannabis, but it doesn’t really deal with whether a 24 year old patient should be allowed to access medical cannabis. As cannabis is often recommended as a second or third line agent, patients have often been exposed to opiates, which carry a far higher rate of addiction.


Moving along into a more recent paper, from 2012: “Increased marijuana use and gender predict poorer cognitive functioning in adolescents and emerging adults.”. This was a relatively small study of 23 marijuana users between the ages of 18–26. There are a number of interesting details in this study. The first is the effect of alcohol; the marijuana group was found to have significantly higher exposure to alcohol than the control group, which could potentially contaminate the results of a neuropsychiatric battery. In addition there was a dose-dependent relationship between memory functioning linked with past year alcohol use, but not marijuana.

The findings of the study were that marijuana users showed cognitive impairment after a week of abstinence in the following areas: sequencing ability/psychomotor speed and cognitive inhibition accuracy. There were no differences found in the areas of verbal memory, selective attention accuracy or verbal and design fluency tasks.

The study’s lead author, Krista Lisdahl: “it’s a mistake for teenagers to use cannabis


The last of these references is a review paper entitled, Long-term consequences of adolescent cannabis exposure on the development of cognition, brain structure and function: an overview of animal and human research, once again focused on…adolescents.

So if all of these articles and authors are primarily raising concerns about the use of cannabis on the brain development of adolescents, why is there a recommendation to not prescribe to patients under the age of 25? When does adolescence end and the age of consent begin?

The last was a bit of a rhetorical question, but I would accept an age definition from the WHO as adolescence lasting from ages 10–19.

This set of guidelines is heavily influenced by a review paper done by Kahan et al. The same recommendation to not prescribe to patients under 25 appears here, this time bolstered by the same references save a change to a different New Zealand study, this one on a cohort of 14–21 year olds.

Cannabis use, and particularly regular or heavy use, was associated with increased rates of a range of adjustment problems in adolescence/ young adulthood-other illicit drug use, crime, depression and suicidal behaviours-with these adverse effects being most evident for school-aged regular users.

Again, reasonable support to be concerned about use in adolescents, but there is nothing here to support a cut off at 25 years of age.

Following back with prior publications from the Kahan, et al cohort (Drs. Spithoff,

youth aged 24 or younger should be strongly encouraged to abstain from or use very little cannabis. — September 2014 (Turner, Spithoff &Kahan)

Things get a little more interesting in this article by Spithoff and Kahan in a Canadian Journal of Addiction special issue on cannabis. Within the text itself:

And finally the under 25 population appears to be a very vulnerable group. The age at which cannabis use becomes safer is unclear; some sources suggest 18, some 21, others 25.

At the conclusion of the article, the authors suggest that physicians support their patients in following previously established low risk use guidelines for cannabis, the first recommendation of which is:

Cannabis use should be delayed until early adulthood (eg 18+ years).

Although if we go back to the original article, it is expressed somewhat differently:

The risks of dependence and other key problems related to use is higher for those who initiate use early, so it would be desirable to delay use until late adolescence (e.g., 16+ years) or better yet early adulthood (e.g., 18+ years).

Heading back to 2006, we get more of a sense of Dr. Kahan’s position, where he takes the “No” position for the question, “Is there a role for marijuana in medical practice?”. In this article he is appropriately concerned about adolescents, though his overall position is that marijuana has no place in medical practice.

Adolescents who smoke cannabis have higher rates of other substance use, school failure, criminal activity, and suicidal thoughts.

So there is still no significant supporting evidence presented here that would recommend an age of 25 to begin cannabis use over 21, or 18. The only new paper that I know of that was not included in this review was from the ALSPAC population in the UK, which looked at the effect of cannabis on children up to the age of 15. This study showed no relationship between moderate adolescent cannabis use and exam results or IQ.

Our findings suggest cannabis may not have a detrimental effect on cognition, once we account for other related factors, particularly cigarette and alcohol use,” Claire Mokrysz, lead researcher from University College London, said in a statement. “This may suggest that previous research findings showing poorer cognitive performance in cannabis users may have resulted from the lifestyle, behavior, and personal history typically associated with cannabis use, rather than cannabis use itself.

This paper likely came out after the guidelines had been drafted but would be useful to include in the updated version.

Clearly this age based recommendation is based more on expert opinion than evidence, especially setting the age at which Canadian patients could access medical cannabis at 25. This age recommendation is ethically unsupportable. It is unconscionable and paternalistic that the CFPC would recommend an age at which a patient who is generally considered competent be refused a treatment based solely on their age. Instead the physician should discuss the risks and benefits of the treatment and then engage in a shared decision making process, as we already do with many other drugs that have far worse side effects.

I would also make a suggestion to the College of Family Practice of Canada — a therapeutics article may be somewhat unbalanced if it is written by someone who feels that there is no place in medical practice for that therapy.

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