Elsevier

Neurotoxicology and Teratology

Volume 27, Issue 2, March–April 2005, Pages 231-239
Neurotoxicology and Teratology

Neurocognitive consequences of marihuana—a comparison with pre-drug performance

https://doi.org/10.1016/j.ntt.2004.11.003Get rights and content

Abstract

In determining the effects of regular marihuana use on neurocognition, abilities within specific relevant cognitive domains prior to regular drug use have not been available. The present study examined effects of current and past regular use of marihuana in subjects for whom pre-drug performance had been ascertained in a prospective, longitudinal fashion. A total of 113 young adults, assessed since infancy, were evaluated using neurocognitive tests for which commensurate measures were obtained prior to the initiation of marihuana smoking. Marihuana users, determined by urinalysis and self-report, were categorized as light(<5 joints per week) and heavy (≥5 joints per week) current users and former users, the latter having used the drug regularly in the past (≥1 joint per week) but not for at least 3 months. A third of the subjects were using marihuana on a regular basis at the time of assessment with half being heavy users. Among former, regular users, approximately half had been smoking 5 or more joints per week. Overall IQ, memory, processing speed, vocabulary, attention, and abstract reasoning were assessed. After accounting for potentially confounding factors and pre-drug performance in the appropriate cognitive domain, current regular heavy users did significantly worse than non-users in overall IQ, processing speed, immediate, and delayed memory. In contrast, the former marihuana smokers did not show any cognitive impairments. It was concluded that residual marihuana effects are evident beyond the acute intoxication period in current heavy users after taking into account pre-drug performance but similar deficits are no longer apparent 3 months after cessation of regular use, even among former heavy using young adults.

Introduction

Objective reports examining the presence and nature of a cognitive dysfunction beyond marihuana's acute intoxication period are equivocal [33], [41]. Most studies that have examined heavy marihuana users for possible long-term neuropsychological impairments assessed subjects after an abstinence period of only a day or two [2], [3], [7], [10], [34], [43]. Among such studies, the areas of vulnerability appear to be memory [43], attention [10], [34], [43], abstract reasoning [34], and processing speed [10]. As cannabinoid metabolites have been identified in the urine of long-term users for weeks following abstinence [9], [11], [27], a negative impact observed following recent use may arise from residue of cannabinoids in the central nervous system (CNS) or may reflect withdrawal effects. In either case, any finding may well be temporary and reversible [5], [6]. On the other hand, CNS alterations noted in individuals abstinent for a sufficient length of time that impairments cannot be attributed to drug residue [41] raise the specter that the cognitive deficits may be irreversible.

Although there is considerable consensus that gross cognitive impairment is not evident in chronic users compared to irregular or non-users following lengthy abstinence periods [13], [29], [36], [40], [42], when specific cognitive deficits are considered, the evidence is inconclusive. A recent, quantitative synthesis of research examining various neurocognitive domains found performance in the spheres of learning and memory to be slightly negatively impacted in former chronic users [25]. However, the authors of this meta-analysis as well as other researchers [19], [23], [33], [42], [43] have emphasized that the pre-drug status of individuals is unknown and, without this knowledge, drug effects may be attributable to some preexisting difference(s). Attempts to deal with this issue include matching procedures on [38], or statistically controlling for [34], a variety of non-marihuana variables. There have also been attempts to estimate pre-drug intellectual functioning by using standardized ability test scores obtained in the fourth grade [2] or contemporary measures of general cognitive functioning that are assumed to be relatively resilient to brain damage [42]. Although these latter approaches are an important improvement, they beg the question of pre-drug functioning in the specific domains of cognitive functioning that appear vulnerable to cannabis use.

As well as better ascertainment of pre-drug cognitive functioning [23], [33], [43], suggestions for improving study designs have emphasized that comparison groups be as similar as possible to drug-using groups [33], [34], [41], [43] and that a prolonged abstinence period is necessary in order to separate drug residual effects from longer lasting neurotoxic CNS effects [23], [33]. A longitudinal, prospective design may serve to address these concerns as such a paradigm permits both within- and between-subject comparisons before, during and after quitting regular marihuana use.

Recently [19], we reported such an approach investigating IQ scores before, during, and after cessation of regular marihuana use in young adults assessed in a host of neurocognitive measures since birth. Using the difference in age-adjusted IQ scores obtained at ages 9–12 compared to scores of the same individuals at 17–20, current marihuana use had a negative effect on global IQ in subjects who were smoking 5 or more joints a week at the time of the latter assessment. However, what aspects within this global measurement might have contributed to the drug effect was not ascertained. It is important to note that although the heavy users in this predominantly middle-class sample experienced a decrease in IQ scores, their scores were still above population norms and, if the preteen IQ scores were not available, these subjects would have appeared to be unaffected. This negative impact was not noted among users of this amount who had stopped smoking marihuana regularly for at least 3 months.

The objective of the present work is to determine whether parallel findings would be observed when specific cognitive functions such as memory, processing speed, abstract reasoning, and attention were evaluated after adjusting for the subjects' pre-drug performance in each of these domains.

Section snippets

Participants

The Ottawa Prenatal Prospective Study (OPPS) was initiated in 1978 with the primary objective of examining the effects on offspring of soft drugs used during pregnancy. In this predominantly middle class, low risk sample, approximately 160 children have been administered neuropsychological tests yearly to age 7 and once during each of the 9–12, 13–16, and 17–21 year intervals. The method of recruitment of women early in their pregnancies, the determination of their drug use [17], and a summary

Sample characteristics

The characteristics of the sample across levels of drug use are described in Table 2.

Although on average, the current and former marihuana using groups showed more positive DSM criteria than the comparison group, the frequency of positive diagnoses for each of the disorders considered was low. To use the diagnostic information on non-drug disorders as a covariate, a dichotomous measure was created whereby each subject with one or more positive diagnoses was assigned a ‘1’ or a ‘0’ if no

Discussion

The results of this study are consistent, in their broad strokes, with much of the extant literature but important differences and extensions exist due to both the longitudinal design of the work permitting the use of pre-drug controls and the nature of many of the outcomes assessed. The present work is the first in which the actual domains measured in young adult current and former users were also evaluated prior to the subjects' onset of regular cannabis use.

The importance of evaluating

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