Our study demonstrates a pattern of gray matter volume changes in a group of regular cannabis users compared with a group of occasional ones. Regular users exhibit a decrease in gray matter (GM) volume in the medial temporal cortex, temporal pole, parahippocampal gyrus, left insula, and orbitofrontal cortex. These changes strongly correlate with the monthly frequency of cannabis use in the 3 months before inclusion in the study. We chose the preceding 3-month period as the period of interest because the participants' cannabis use during this length of time is representative of their usual cannabis use. Three clusters in the cerebellum show the opposite behavior, with increased GM volume. We also show that the age of onset of cannabis use is correlated with the magnitude of gray matter volume reduction in the cerebral hemispheres. Specifically, significant gray matter atrophy can occur either with a heavy cannabis consumption independent of the age of first use or with recreational consumption that started during adolescence (before the age of 18).
