As yet, there are only two major prospective cohort studies of the effects of marijuana on cognition that have data available on cognition in children years before they abuse marijuana (15, 16). These studies adjusted for potential confounds such as other drug use, comorbid psychiatric disorder, socioeconomic status, and parental drug use. Both studies found a significant decline in IQ from childhood to adulthood among regular users, defined as four or five “joints” or occasions of marijuana use per week, a pattern of exposure similar to that of the monkeys in the current study by Verrico and colleagues (1). In the earlier study, only those teens (ages 17–20) who were current regular users of marijuana had a decline in IQ, while former regular users had a normal gain in IQ over the prior 8 years, despite greater lifetime exposure to marijuana (15). In the larger, more recent cohort study, a decline in IQ from childhood (ages 7 to 13) to middle adulthood (age 38) was found for persistent users, most of whom met criteria for cannabis dependence (16). Among these adult persistent users, cessation of marijuana use apparently did not reverse decline in IQ, particularly if the onset of use was in adolescence. Together, these studies suggest there is a window of recovery, such that teens who use marijuana regularly have the opportunity to restore cognition only if they can achieve abstinence soon thereafter.