Deficits were found across multiple cognitive domains.

To examine the relationship between adolescent- or adult-onset persistent cannabis use and cognitive changes, investigators analyzed prospective data on the 1037 participants in the longitudinal Dunedin Study. The data included interviews at ages 18, 21, 26, 32, and 38; IQ testing in childhood (biannually at ages 7–13) and at age 38; and neuropsychological tests at age 38. Only seven participants reported any cannabis use by age 13.

Compared with adult-onset persistent users, adolescent-onset persistent users with at least three cannabis dependence diagnoses had an average 8-point decline in IQ by age 38. Regardless of dependence diagnoses, adolescent-onset users showed statistically significant impairments across multiple domains of cognitive functioning. Informants, who were chosen by the participants, reported decrements in attention and memory functioning at age 38 among those who began using cannabis in adolescence. Results remained significant when controlled for alcohol, tobacco, and hard drug intake; cannabis use at the time of testing; and schizophrenia diagnoses. Earlier and more-intensive use was associated with greater cognitive impairment. Among adolescent-onset users who had stopped use 1 year before testing, neuropsychological outcomes did not fully improve.

Comment: The belief that long-term cannabis represents a more benign recreational drug than alcohol is belied by this research and by reports of increased vulnerability to psychosis in adolescent-onset users (Acta Psychiatr Scand 2012; 125:45). Clinicians treating adolescent patients need to inquire specifically about marijuana, because teenagers may not be aware that its use can be associated with intellectual or psychopathological impairment.

Source: Journal Watch Psychiatry