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.