Those who use cannabis (as opposed to having used it in the past) may be more likely to have short-term memory problems.

Long-term use of marijuana has been found to affect verbal fluency, verbal memory, attention, and psychomotor speed, with longer use correlating with greater impairment. However, although there have been a number of studies into the long-term effects of cannabis of memory and learning, most are small or not sufficiently rigorous. We cannot, therefore, make any clear conclusions on this topic. Verbal memory appears to be the specific type of memory that is particularly vulnerable, but there is no clear evidence of long-term damage.

It may be that the effects of cannabis on cognition depends on various factors. For example, there is some evidence that starting in adolescence may be more likely to produce impairment, some suggestion that it depends on the strain of cannabis (there’s some evidence that cannabidiol can counteract the memory-impairing effects of the psychoactive component THC), a possibility that certain conditions could interact (one study found that MS sufferers who used marijuana for pain relief performed significantly worse on tests of attention, speed of thinking, executive function and visual perception of spatial relationships between objects).

Chronic use of alcohol and marijuana during youth has been associated with poorer neural and cognitive function, which appears to continue into adulthood. A new study looking specifically at white-matter changes provides more support for the idea that adolescent brains may be at particular risk from the damage that substance abuse can bring.

The brain-imaging study compared 41 adolescents (aged 16-20) with extensive marijuana- and alcohol-use histories by mid-adolescence with 51 adolescents with no such history. The study found that substance users showed poorer white matter integrity in seven tracts (right and left superior longitudinal fasciculus, right posterior thalamic radiations, right prefrontal thalamic fibers, right superior temporal gyrus white matter, right inferior longitudinal fasciculus, left posterior corona radiata).

Two brain scans were taken, at baseline and at 18 months. Substance use interviews were given every six months.

More alcohol use during the interval was associated with worse integrity in both the right and left superior longitudinal fasciculi, above and beyond baseline values in these bundles. Marijuana use didn’t predict change over time. Those who had a history of more risk-taking behaviors showed poorer integrity of the right prefrontal thalamic fibers.

The findings add to previous research showing white matter problems in youth with substance-use histories. The study points to alcohol use during adolescence being particularly problematic. It also suggests that youth who engage in risk-taking behaviors may tend to have poorly developed fronto-thalamic tracts.

All of this is particularly worrying because it is thought that maturation of the brain during adolescence is the foundation for self-control, suggesting that substance abuse during this period may have long-lasting effects on the individual’s ability to plan, organize, and self-regulate.

A large long-running New Zealand study has found that people who started using cannabis in adolescence and continued to use it for years afterward showed a significant decline in IQ from age 13 to 38. This was true even in those who hadn’t smoked marijuana for some years.

The study has followed a group of 1,037 children born in 1972-73. At age 38, 96% of the 1004 living study members participated in the latest assessment. Around 5% were regularly smoking marijuana more than once a week before age 18 (cannabis use was ascertained in interviews at ages 18, 21, 26, 32, and 38 years, and this group was not more or less likely to have dropped out of the study).

This group showed an average decline in IQ of 8 points on cognitive tests at age 38 compared to scores at age 13. Such a decline was not found in those who began using cannabis after the age of 18. In comparison, those who had never used cannabis showed a slight increase in IQ. The effect was dose-dependent, with those diagnosed as cannabis dependent on three or more occasions showing the greatest decline.

While executive function and processing speed appeared to be the most seriously affected areas, impairment was seen across most cognitive domains and did not appear to be statistically significantly different across them.

The size of the effect is shown by a further measure: informants (nominated by participants as knowing them well) also reported significantly more attention and memory problems among those with persistent cannabis dependence. (Note that a decline of 8 IQ points in a group whose mean is 100 brings it down to 92.)

The researchers ruled out recent cannabis use, persistent dependence on other drugs (tobacco, alcohol, hard drugs), and schizophrenia, as alternative explanations for the effect. The effect also remained after years of education were taken into account.

The finding supports the view that the adolescent brain is vulnerable to the effects of marijuana, and that these effects are long-lasting and significant.

Some numbers for those interested: Of the 874 participants included in the analysis (those who had missed at least 3 interviews in the 25 years were excluded), 242 (28%) never used cannabis, 479 (55%) used it but were never diagnosed as cannabis-dependent, and 153 (17%) were diagnosed on at least one of the interviews as cannabis-dependent. Of these, 80 had been so diagnosed on only one occasion, 35 on two occasions, and 38 on three or more occasions. I note that the proportion of males was significantly higher in the cannabis-dependent groups (39% in never used; 49% in used but never diagnosed; 70%, 63%, 82% respectively for the cannabis-dependent).

A new study explains how marijuana impairs working memory. The component THC removes AMPA receptors for the neurotransmitter glutamate in the hippocampus. This means that there are fewer receivers for the information crossing between neurons.

The research is also significant because it adds to the growing evidence for the role of astrocytes in neural transmission of information.

This is shown by the finding that genetically-engineered mice who lack type-1 cannabinoid receptors in their astroglia do not show impaired working memory when exposed to THC, while those who instead lacked the receptors in their neurons do. The activation of the cannabinoid receptor expressed by astroglia sends a signal to the neurons to begin the process that removes AMPA receptors, leading to long-term depression (a type of synaptic plasticity that weakens, rather than strengthens, neural connections).

See the Guardian and Scientific American articles for more detail on the study and the processes involved.

For more on the effects of marijuana on memory

Research into the effects of cannabis on cognition has produced inconsistent results. Much may depend on extent of usage, timing, and perhaps (this is speculation) genetic differences. But marijuana abuse is common among sufferers of schizophrenia and recent studies have shown that the psychoactive ingredient of marijuana can induce some symptoms of schizophrenia in healthy volunteers.

Now new research helps explain why marijuana is linked to schizophrenia, and why it might have detrimental effects on attention and memory.

In this rat study, a drug that mimics the psychoactive ingredient of marijuana (by activating the cannabinoid receptors) produced significant disruption in brain networks, with brain activity becoming uncoordinated and inaccurate.

In recent years it has become increasingly clear that synchronized brainwaves play a crucial role in information processing — especially that between the hippocampus and prefrontal cortex (see, for example, my reports last month on theta waves improving retrieval and the effect of running on theta and gamma rhythms). Interactions between the hippocampus and prefrontal cortex seem to be involved in working memory functions, and may provide the mechanism for bringing together memory and decision-making during goal-directed behaviors.

Consistent with this, during decision-making on a maze task, hippocampal theta waves and prefrontal gamma waves were impaired, and the theta synchronization between the two was disrupted. These effects correlated with impaired performance on the maze task.

These findings are consistent with earlier findings that drugs that activate the cannabinoid receptors disrupt the theta rhythm in the hippocampus and impair spatial working memory. This experiment extends that result to coordinated brainwaves beyond the hippocampus.

Similar neural activity is observed in schizophrenia patients, as well as in healthy carriers of a genetic risk variant.

The findings add to the evidence that working memory processes involve coordination between the prefrontal cortex and the hippocampus through theta rhythm synchronization. The findings are consistent with the idea that items are encoded and indexed along the phase of the theta wave into episodic representations and transferred from the hippocampus to the neocortex as a theta phase code. By disrupting that code, cannabis makes it more difficult to retain and index the information relevant to the task at hand.

A study involving 50 people with MS (aged 18-65), of whom half used marijuana for pain relief, has found that marijuana users performed significantly worse on tests of attention, speed of thinking, executive function and visual perception of spatial relationships between objects. Those who used marijuana were also twice as likely as non-users to be classified as globally cognitively impaired.

The two groups were matched for age, gender, level of education, IQ before diagnosis, level of disability and duration of time with MS. On average, the duration of marijuana use was 26 years, and 72% reported smoking marijuana on a daily basis while 24% reported weekly use and one person reported bi-weekly use. There were no differences between the groups on measures of depression and anxiety.

And on a less-expected note, a study involving 40 people with multiple sclerosis and 40 people without MS has found those with MS scored 70% better on cognitive tests during cooler days compared to warmer days of the year. There was no link between test scores and temperature for those without MS.

[2178] Honarmand K, Tierney MC, O'Connor P, Feinstein A. Effects of cannabis on cognitive function in patients with multiple sclerosis. Neurology [Internet]. 2011 ;76(13):1153 - 1160. Available from:

The findings of the temperature study were presented at the American Academy of Neurology's 63rd Annual Meeting in Honolulu April 9 to April 16, 2011.

A study involving 48 adolescents, of whom 19 had been diagnosed with substance abuse/dependence, and 14 had a family history of substance abuse but no history of personal use, has found that greater alcohol use was associated with a significant decrease in attention and executive function (which is involved in planning and decision-making), while greater marijuana use was associated with poorer memory. Adolescents in the substance abuse group had lower scores in attention, memory, and processing speed, compared to the other groups, while those with a family history of abuse (but no personal history) had poorer visuospatial ability.

A study involving 134 cannabis users aged 16-23 has found that when they were smoking cannabis containing a low percentage of cannabidiol they performed much worse on the memory tests. In contrast, those smoking cannabis high in cannabidiol performed just as well on the tests when they were intoxicated as when they were sober. There were no differences in the THC content of the cannabis smoked by any of the participants (THC is the main psychoactive ingredient, which gives the characteristic ‘stoned’ feeling, and feelings of paranoia).

For the study, the participants were tested on two separate occasions — once while they were smoking their own preferred type of cannabis and were intoxicated, and once when they had not smoked for the last 24 hours and were sober.

Levels of cannabidiol in cannabis can range from virtually none to 40%. This study suggests that cannabidiol can counteract the memory-impairing effects of THC. Unfortunately, low-cannabidiol strains (like skunk) now dominate the market of street cannabis, suggesting current users will be more at risk of cognitive impairment.

Older news items (pre-2010) brought over from the old website

Long-term cannabis users may have structural brain abnormalities

An imaging study of 15 men who smoked more than five cannabis joints daily for more than 10 years has found that, compared with individuals who were not cannabis users, the heavy cannabis users tended to have a smaller hippocampus and amygdala. They also performed significantly worse on verbal learning, but this didn’t correlate with regional brain volumes.

[908] Yucel M, Solowij N, Respondek C, Whittle S, Fornito A, Pantelis C, Lubman DI. Regional Brain Abnormalities Associated With Long-term Heavy Cannabis Use. Arch Gen Psychiatry [Internet]. 2008 ;65(6):694 - 701. Available from:

New imaging technique finds no damage to adolescent brains of moderate cannabis users

A preliminary study using a new imaging technique to compare the brain tissue of young people who had used cannabis moderately as teenagers and young people who had not, has failed to find any indication that damage to the developing adolescent brain occurred.

[721] DeLisi L, Bertisch H, Szulc K, Majcher M, Brown K, Bappal A, Ardekani B. A preliminary DTI study showing no brain structural change associated with adolescent cannabis use. Harm Reduction Journal [Internet]. 2006 ;3(1):17 - 17. Available from:

Memory and speed of thinking get worse over time with marijuana use

A study of 20 long-term marijuana users, 20 shorter-term users and 24 control subjects who had used marijuana at least once in their lives but not more than 20 times and not in the past two years, found that the longer people used marijuana, the more deterioration they had in verbal fluency, verbal memory, attention, and psychomotorspeed. For example, in a test where participants needed to remember a list of words that had been read to them earlier, the non-users remembered an average of 12 out of 15 words, the shorter-term users remembered an average of nine words and the long-term users remembered an average of seven words.

[1322] Messinis L, Kyprianidou A, Malefaki S, Papathanasopoulos P. Neuropsychological deficits in long-term frequent cannabis users. Neurology [Internet]. 2006 ;66(5):737 - 739. Available from:

Minimal long-term effects of marijuana use on learning and memory

An analysis of 15 previously published research studies on the non-acute (residual) effects of cannabis on the neurocognitive performance of adult long-term, recreational users of marijuana has failed to reveal a substantial, systematic effect on the neurocognitive functioning of users, although there was a very small effect in learning new information. The researchers noted however, that many of the research studies examined had significant limitations, either with small numbers of subjects or insufficient information about potential confounding factors, such as exposure to other drugs or presence of neuropsychiatric factors such as depression or personality disorders.

[2384] Grant I, Gonzalez R, Carey CL, Natarajan L, Wolfson T. Non-Acute (residual) Neurocognitive Effects of Cannabis Use: A Meta-Analytic Study. Journal of the International Neuropsychological Society [Internet]. 2003 ;9(05):679 - 689. Available from:

Cognitive impairment seen in heavy marijuana users who started using before 18

A study of 122 long-term heavy users of marijuana and 87 subjects who had used marijuana only a few times (control subjects), all of whom had refrained from any drug use 28 days prior to testing, found that those who had started using marijuana at age 17 or younger performed significantly worse on the tests assessing verbal functions such as verbal IQ and memory of word lists than did those who started using marijuana later in life or who had used the drug sparingly. There were virtually no differences in test results among the individuals who started marijuana use after age 17 and the control subjects. The subjects were all between 30 and 55 years of age.

[510] Pope HG, Gruber AJ, Hudson JI, Cohane G, Huestis MA, Yurgelun-Todd D. Early-onset cannabis use and cognitive deficits: what is the nature of the association?. Drug and Alcohol Dependence [Internet]. 2003 ;69(3):303 - 310. Available from: