Pope and colleagues

We recruited individuals aged 30 to 55 years in 3 groups: (1) 63 current heavy cannabis users who had smoked cannabis at least 5000 times in their lives and who were smoking daily at study entry; (2) 45 former heavy users who had also smoked at least 5000 times but fewer than 12 times in the last 3 months; and (3) 72 control subjects who had smoked no more than 50 times in their lives. Subjects underwent a 28-day abstinence from cannabis use, monitored by observed urine samples. On days 0, 1, 7, and 28, they completed a complex neuropsychological test battery.
Results: At days 0, 1, and 7, current heavy users scored significantly below control subjects on recall of word lists. By day 28, however, there were virtually no significant differences among the groups on any of the test results, and no significant associations between cumulative lifetime cannabis use and test scores.
Conclusion: Some cognitive deficits appear detectable at least 7 days after heavy cannabis use but appear reversible and related to recent cannabis exposure rather than irreversible and related to cumulative lifetime use.
Modified according to: Pope HG Jr, et al. Neuropsychological performance in long-term cannabis users. Arch Gen Psychiatry 2001 Oct;58(10):909-15.
 

Peter Fried and colleagues

We determined marijuana use for seventy 17- to 20-year-olds whose intelligence quotient (IQ) had been determined at the age of 9-12 years. The IQ difference scores were calculated by subtracting each person's IQ score at 9-12 years (before initiation of drug use) from his or her score at 17-20 years. We then compared the difference in IQ scores of current heavy users (at least 5 joints per week), current light users (less than 5 joints per week), former users (who had not smoked regularly for at least 3 months) and non-users (who never smoked more than once per week and no smoking in the past two weeks).
Results: Current marijuana use was significantly correlated in a dose-related fashion with a decline in IQ over the ages studied. The comparison of the IQ difference scores showed an average decrease of 4.1 points in current heavy users compared to gains in IQ points for light current users (5.8), former users (3.5) and non-users (2.6).
Interpretation: Current marijuana use had a negative effect on global IQ score only in subjects who smoked 5 or more joints per week. A negative effect was not observed among subjects who had previously been heavy users but were no longer using the substance. We conclude that marijuana does not have a long-term negative impact on global intelligence. Whether the absence of a residual marijuana effect would also be evident in more specific cognitive domains such as memory and attention remains to be ascertained.
Modified according to: Fried P, et al. Current and former marijuana use: preliminary findings of a longitudinal study of effects on IQ in young adults. CMAJ 2002;166(7):887-91.
 

Nadia Solowij & Brin Greyner

Cannabis exerts its most prominent effects on the central nervous system (CNS), whether smoked or ingested. It is primarily for its mind altering or psychoactive properties that it is used recreationally in many parts of the world. Human studies of the acute effects of cannabis suggest that the cannabinoid receptor system may be involved in regulating mood, emotion, attention, memory and many other cognitive functions. What is not yet certain is the extent to which any of these functions, and indeed the endogenous cannabinoid system and receptor itself, are affected by the prolonged use of exogenous cannabinoids. The evidence to date from both human and animal research suggests that they are not grossly impaired in the long term but that there are alterations in their function. (…)
It is not clear to what extent the cognitive and psychological effects of long term cannabis use might impact upon daily life, although cannabis users themselves complain of problems with memory, concentration, loss of motivation, paranoia, depression, dependence and lethargy. Schwenk (1998) has argued that there is no clear causal relationship between cannabis use and job performance. The nature of the cognitive deficits as assessed by psychological testing suggests that long term users would perform reasonably well in routine tasks of everyday life, although they may be more distractible. Difficulties are likely to be encountered in performing complex tasks that are novel or that cannot be solved by automatic application of previous knowledge, or with tasks that rely heavily on a memory component or require strategic planning and multi-tasking. (…) The extent to which the subtle cognitive impairments may recover following cessation of use is also unknown, but research is in progress.
Solowij N, Greyner B. Long term effects of cannabis on psyche and cognition. In: Grotenhermen F, Russo E, eds. Cannabis and cannabinoids: pharmacology, toxicology and therapeutic potential. Binghamton, NY: Haworth Press, 2001, in press.
 

Lynn Zimmer & John Morgan

The cognitive process most clearly affected by marijuana is short-term memory. In laboratory studies, subject under the influence of marijuana have no trouble remembering things they learned previously. However, they display diminished capacity to learn and recall new information. This diminishment lasts for the duration on intoxication. There is no convincing evidence that heavy long-term marijuana use permanently impairs memory or other cognitive functions. (...)
During the past thirty years, researchers have found, at most, minor cognitive differences between chronic marijuana users and non-users, and the results differ substantially from one study to another. Based on this evidence, it does not appear that long-term marijuana use causes significant permanent harm to intellectual ability.
Zimmer L, Morgan JP. Marijuana Myths Marijuana Facts. A review of the scientific evidence. New York/San Francisco: The Lindesmith Center, 1997.
 

House of Lords (UK)

Cannabis can have untoward long-term effects on cognitive performance, i.e. the performance of the brain, particularly in heavy users. These have been reviewed for us by the Royal College of Psychiatrists and the Royal Society. While users may show little or no impairment in simple tests of short-term memory, they show significant impairments in tasks that require more complex manipulation of learned material (so-called "executive" brain functions). There is some evidence that some impairment in complex cognitive function may persist even after cannabis use is discontinued; but such residual deficits if present are small, and their presence controversial. Dr Jan van Amsterdam of the Netherlands National Institute of Public Health and the Environment, who has reviewed the literature on long-term cognitive effects of prolonged heavy use and kindly came to Westminster to tell us his findings, pointed out the practical difficulties of assessing possible residual effects. These include the impossibility of obtaining predrug baseline values (i.e. measures of the cognitive functioning of the subject before their first use of cannabis), the difficulty of estimating the drug dose taken, the need for a lengthy "washout" period after termination of use to allow for the slow elimination of residual cannabis from the body, and the possibility of confusing long-term deficits with withdrawal effects. He felt that many of the published reports on this subject had not taken adequate account of these problems.
House of Lords Select Committee on Science and Technology. Cannabis. The scientific and medical evidence. London: The Stationery Office, 1998.