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.
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.
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.
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.
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).
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.
Yucel, M., Solowij, N., Respondek, C., Whittle, S., Fornito, A., Pantelis, C., & Lubman, D. I. (2008). Regional Brain Abnormalities Associated With Long-term Heavy Cannabis Use. Arch Gen Psychiatry, 65(6), 694-701. Retrieved from http://archpsyc.ama-assn.org/cgi/content/abstract/65/6/694
http://www.eurekalert.org/pub_releases/2008-06/usmc-usr061208.php
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.
DeLisi, L., Bertisch, H., Szulc, K., Majcher, M., Brown, K., Bappal, A., & Ardekani, B. (2006). A preliminary DTI study showing no brain structural change associated with adolescent cannabis use. Harm Reduction Journal, 3(1), 17-17. Retrieved from http://www.harmreductionjournal.com/content/3/1/17
http://www.sciencedaily.com/releases/2006/05/060510092242.htm
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 psychomotor speed. 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.
Messinis, L., Kyprianidou, A., Malefaki, S., & Papathanasopoulos, P. (2006). Neuropsychological deficits in long-term frequent cannabis users. Neurology, 66(5), 737-739. Retrieved from http://www.neurology.org/cgi/content/abstract/66/5/737
http://www.eurekalert.org/pub_releases/2006-03/aaon-mso030706.php
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.
Grant, I., Gonzalez, R., Carey, C. L., Natarajan, L., & Wolfson, T. (2003). Non-Acute (residual) Neurocognitive Effects of Cannabis Use: A Meta-Analytic Study. Journal of the International Neuropsychological Society, 9(05), 679-689. Retrieved from http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=154103&fulltextType=RA&fileId=S1355617703950016
http://www.eurekalert.org/pub_releases/2003-06/uoc--mle062603.php
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.
Pope, H. G., Gruber, A. J., Hudson, J. I., Cohane, G., Huestis, M. A., & Yurgelun-Todd, D. (2003). Early-onset cannabis use and cognitive deficits: what is the nature of the association? Drug and Alcohol Dependence, 69(3), 303-310. Retrieved from http://www.sciencedirect.com/science/article/B6T63-47HC4M7-5/2/b079015351810194445d2e55f324afec
http://www.eurekalert.org/pub_releases/2003-05/niod-smu050203.php
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.
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.
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).