Long-term use of cannabis (marijuana) may lead to cognitive deficits, suggests a large observational study published in the American Journal of Psychiatry in May. Using data from a cohort of more than 1,000 people born in New Zealand in 1972–1973 and followed until age 45, investigators found declines in IQ from childhood (before cannabis use started) to midlife (of 5.5 points on average) in those who used cannabis long term (several years or more) and heavily (at least weekly, with most people using it four times or more a week). Non-users did not show IQ decline. Cannabis users also exhibited poorer learning, slower processing speeds, and more attention problems at age 45 compared to non-users.

Such deficits were not seen or were present to a lesser degree in longtime tobacco and alcohol users, those who used cannabis recreationally in midlife, and those who quit cannabis. The researchers also controlled for other factors that can affect cognitive functioning, including other illicit drug use, low socioeconomic status in childhood, and a family history of substance abuse.

In addition, long-term cannabis users showed more shrinkage in their hippocampus, a part of the brain that is involved in memory and learning. More research is needed to determine whether cannabis use increases dementia later in life, though mild cognitive deficits and increased hippocampal atrophy in midlife are risk factors.