Long-term cannabis users showed deficits in cognition and smaller hippocampal volume by midlife, according to results of a New Zealand study published in the American Journal of Psychiatry.

“It is unclear whether the subtle cognitive and brain differences observed in young cannabis users might be larger in midlife and older adult cannabis users with longer histories of use,” Madeline H. Meier, PhD, of the department of psychology at Arizona State University, and colleagues wrote.

Meier and colleagues wanted to see if long-term cannabis use is associated with cognitive deficits and smaller hippocampal volume by middle age, and how those factors are associated with risk for dementia later in life.

The study involved a representative cohort of 1,037 people born in Dunedin, New Zealand, between April 1972 and March 1973, who were followed through the age of 45 years.

Participants were assessed for cannabis use and dependence at ages 18, 21, 26, 32, 38 and 45, with IQ gauged at ages 7, 9, 11 and 45 years. Assessments of specific neuropsychological functions and hippocampal volume were undertaken for all participants at age 45 years. Ninety-four percent of participants at baseline were retained through the entire study interval.

Results showed that long-term cannabis users’ IQ declined from childhood to midlife (mean = 5.5 IQ points), with resultant poorer learning and processing speed relative to their childhood IQ, as well as memory and attention problems.

Cognitive deficits among the study group could not be explained by habitual use of tobacco, alcohol or other illicit drugs; socioeconomic status in younger years or family history of substance dependence. Data also revealed that long-term cannabis users had reduced hippocampal volume, but it did not statistically mediate cannabis-related cognitive deficits.

“The deficits we observed are comparable to midlife cognitive deficits of individuals who developed dementia in the Atherosclerosis Risk in Communities Study,” Meier and colleagues wrote. “It is timely given the confluence of two trends: the growth of the aging population, and the record high rates of cannabis use among today’s older adults.”