A large prospective, longitudinal study showed long-term cannabis users had an intelligence quotient (IQ) decline from age 18 to midlife (mean, 5.5 IQ points), poorer learning and processing speed, compared with childhood, and self-reported memory and attention problems. Long-term cannabis users also showed hippocampal atrophy at midlife (age 45), which combined with mild midlife cognitive deficits, all known risk factors for dementia.
“Long-term cannabis users – people who have used cannabis from 18 or 19 years old and continued using through midlife – showed cognitive deficits, compared with nonusers. They also showed more severe cognitive deficits, compared with long-term alcohol users and long-term tobacco users. But people who used infrequently or recreationally in midlife did not show as severe cognitive deficits. Cognitive deficits were confined to cannabis users,” lead investigator Madeline Meier, PhD, associate professor of psychology, Arizona State University, Tempe, said in an interview.
“Long-term cannabis users had smaller hippocampal volume, but we also found that smaller hippocampal volume did not explain the cognitive deficits among the long-term cannabis users,” she added.
The study was recently published online in the American Journal of Psychiatry.
Growing use in Boomers
Long-term cannabis use has been associated with memory problems. Studies examining the impact of cannabis use on the brain have shown conflicting results. Some suggest regular use in adolescence is associated with altered connectivity and reduced volume of brain regions involved in executive functions such as memory, learning, and impulse control compared with those who do not use cannabis.
Others found no significant structural differences between the brains of cannabis users and nonusers.
An earlier, large longitudinal study in New Zealand found that persistent cannabis use (with frequent use starting in adolescence) was associated with a loss of an average of six (or up to eight) IQ points measured in mid-adulthood.
Cannabis use is increasing among Baby Boomers – a group born between 1946 and 1964 – who used cannabis at historically high rates as young adults, and who now use it at historically high rates in midlife and as older adults.
To date, case-control studies, which are predominantly in adolescents and young adults, have found that cannabis users show subtle cognitive deficits and structural brain differences, but it is unclear whether these differences in young cannabis users might be larger in midlife and in older adults who have longer histories of use.
The study included a representative cohort of 1,037 individuals in Dunedin, New Zealand, born between April 1972 and March 1973, and followed from age 3 to 45.
Cannabis use and dependence were assessed at ages 18, 21, 26, 32, 38, and 45. IQ was assessed at ages 7, 9, 11, and 45. Specific neuropsychological functions and hippocampal volume were assessed at age 45.
“Most of the previous research has focused on adolescent and young-adult cannabis users. What we’re looking at here is long-term cannabis users in midlife, and we’re finding that long-term users show cognitive deficits. But we’re not just looking at a snapshot of people in midlife, we’re also doing a longitudinal comparison – comparing them to themselves in childhood. We saw that long-term cannabis users showed a decline in IQ from childhood to adulthood,” said Dr. Meier.
Participants in the study are members of the Dunedin Longitudinal Study, a representative birth cohort (n = 1,037; 91% of eligible births; 52% male) born between April 1972 and March 1973 in Dunedin, New Zealand, who participated in the first assessment at age 3.
This cohort matched socioeconomic status (SES), key health indicators, and demographics. Assessments were carried out at birth and ages 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, 32, 38, and 45. IQ was assessed at ages 7, 9, 11, and 45. Specific neuropsychological functions and hippocampal volume were assessed at age 45.