Associations between Adolescent Cannabis Use and Neuropsychological Decline : A Longitudinal Co-Twin Control Study
Madeline H. Meier, Avshalom Caspi, Andrea Danese, Helen L. Fisher, Renate Houts, Louise Arseneault, & Terrie E. Moffitt
Addiction. 2018, 113, (2), 257-265.
Aims : This study tested whether adolescents who used cannabis or met criteria for cannabis dependence showed neuropsychological impairment prior to cannabis initiation and neuropsychological decline from before to after cannabis initiation.
Design : A longitudinal co-twin control study.
Setting and Participants : Participants were 1,989 twins from the Environmental Risk (E-Risk) Longitudinal Twin Study, a nationally representative birth cohort of twins born in England and Wales from 1994-1995.
Measurements : Frequency of cannabis use and cannabis dependence were assessed at age 18. Intelligence quotient (IQ) was obtained at ages 5, 12, and 18. Executive functions were assessed at age 18.
Findings : Adolescents who used cannabis had lower IQ in childhood, prior to cannabis initiation, and had lower IQ at age 18, but there was little evidence that cannabis use was associated with IQ decline from age 12-18. For example, adolescents with cannabis dependence had age-12 and age-18 IQ scores that were 5.61 (t=-3.11, p=.002) and 7.34 IQ points (t=-5.27, p<.001) lower than adolescents without cannabis dependence, but adolescents with cannabis dependence did not show greater IQ decline from age 12-18 (t=-1.27, p=.20). Moreover, adolescents who used cannabis had poorer executive functions at age 18, but these associations were generally not apparent within twin pairs. For example, twins who used cannabis more frequently than their co-twin performed similarly to their co-twin on 5 of 6 executive function tests (ps>.10). The one exception was that twins who used cannabis more frequently than their co-twin performed worse on one working memory test (Spatial Span Reversed; β=-0.07, p=.036).
Conclusions : Family background factors explain why adolescent cannabis users perform worse on IQ and executive function tests. Therefore, short-term cannabis use in adolescence does not appear to cause IQ decline or impair executive functions, even when cannabis use reaches the level of dependence.
Debate concerning cannabis legalization has led to increased urgency to understand the effects of cannabis use on health and behavior (1). The effect of cannabis use on neuropsychological functions has received considerable research attention, and the general consensus is that heavy cannabis use is associated with neuropsychological impairment (2-7). However, there is uncertainty regarding the extent to which neuropsychological impairment is apparent prior to cannabis use initiation, the age at which cannabis-related neuropsychological impairment first emerges, and the level and duration of cannabis exposure that is sufficient to produce impairment. One hypothesis is that neuropsychological impairment is apparent in childhood, prior to cannabis use initiation. A second hypothesis is that cannabis-induced neuropsychological impairment first emerges in adolescence shortly after cannabis use initiation. Yet a third hypothesis is that cannabis-induced neuropsychological impairment emerges only after years of heavy use. Determining which hypothesis has more support will have critical implications for prevention and remediation.
To address these questions, prospective longitudinal studies are needed. There are only nine cohort studies of the association between cannabis use and neuropsychological impairment that could inform these questions. These studies included adolescents or young adults in the sample and administered neuropsychological tests at two or more time points (Supplemental Table 1). Six of these studies assessed neuropsychological functions in childhood, prior to cannabis use initiation, and therefore had ‘before and after’ assessments of neuropsychological functions (8, 11, 12, 14-17). These six studies found inconsistent evidence for the hypothesis that neuropsychological impairment predates cannabis initiation (8, 11, 12, 14-17).
Across all nine studies, there was mixed evidence that cannabis use was associated with neuro-psychological decline (or neuropsychological impairment after accounting for baseline neuro-psychological functioning). However, studies varied in terms of length of follow-up and the cohorts’ level of cannabis exposure. In general, studies with the longest follow-up (8, 9) and greatest cannabis exposure (8, 9, 11) tended to show the strongest evidence of cannabis-related neuropsychological decline, and studies with the shortest follow-up period and least cannabis exposure (14-17) (i.e., studies of adolescent cannabis use) tended to show the weakest evidence. This pattern is consistent with the hypothesis that cannabis-induced neuropsychological impairment emerges only after years of heavy cannabis use. Nonetheless, firm conclusions cannot be drawn for several reasons. First, there are relatively few cohort studies, particularly studies that assessed neuropsychological functions prior to cannabis initiation. Second, existing cohort studies of adolescents examined low-level cannabis use (14-17), leaving open the possibility that neuropsychological impairment might emerge only for adolescents with more problematic use. Third, there are many potential confounders of cannabis-neuropsychological impairment associations, limiting causal inference.
The purpose of the present study was to test associations between adolescent cannabis use and neuropsychological decline in a cohort of British children followed prospectively from age 5-18. Like the few existing cohort studies of adolescent cannabis use (11, 14-17), we assessed intelligence (IQ) in childhood, prior to cannabis use initiation. We also assessed IQ and executive functions at age 18, after some cohort members had begun using cannabis. Unlike other cohort studies of adolescent cannabis use (11, 14-16), we examined cannabis dependence as our cannabis exposure, in addition to frequency of cannabis use, as cannabis dependence is an indicator of more problematic use. Further, because the cohort comprises twin pairs, it enabled a comparison of neuropsychological decline for twins in the same family who differed in their cannabis use. This within-pair comparison is important because it controls for family background factors that might lead to a spurious association between cannabis use and neuropsychological decline.