Compared to some of the drugs out there, cannabis can seem relatively harmless. It doesn’t have the ruinous effects of methamphetamines or even substances like synthetic pot. But there has long been suspicion that heavy use might have long-term effects on IQ, for instance [pdf].
Factors that tend to accompany cannabis consumption, such as the use of other drugs and alcohol and, in adolescents, a tendency to skip class, have made it difficult to decisively pin a dip in IQ to marijuana use. To clear away the noise, the authors of a new study in the Proceedings of the National Academy of Sciences turned to the reams of data from the Dunedin Multidisciplinary Health and Development Study, and they’ve found that on average, by the time they reached age 38, heavy pot users diagnosed with cannabis dependence during adolescence suffered an 8-point drop in IQ.
In the decades-long Dunedin study, originally designed to test the efficacy of new neonatal intensive care units, 1,037 babies born in Dunedin, New Zealand, between 1972 and 1973 became some of the most observed people you’ve probably never heard of. Researchers assessed the children every few years until …
from Discover Magazine