“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 non-users.
Long-term cannabis use has been associated with memory problems. Studies examining the impact of cannabis use on the brain have shown conflicting results.
Cannabis use and dependence were assessed at ages 18, 21, 26, 32, 38, and 45.
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.
Assessments were carried out at birth and ages 3, 5, 7, 9, 11, 13, 15, 18, 21, 26, 32, 38, and 45.
The hippocampus was the area of focus because it has a high density of cannabinoid receptors and is also instrumental for learning and memory, which is one of the most consistently impaired cognitive domains in cannabis users, and has been the brain region that most consistently emerges as smaller in cannabis users relative to controls.
Cognitive functioning among midlife recreational cannabis users was similar to representative cohort norms, suggesting that infrequent recreational cannabis use in midlife is unlikely to compromise cognitive functioning.
And the deficits we saw among long-term cannabis users , they were in the range in terms of effect size of what we see among people in other studies who have gone on to develop dementia in later life,” said Meier.
The study findings conflict with those of other studies, including one by the same research group, which compared the cognitive functioning of twins who were discordant for cannabis use and found little evidence of cannabis-related cognitive deficits.
“Group sizes were small but we didn’t rely only on those group comparisons; however, we did find statistical differences.
“These dose-response associations used large sample sizes, were highly powered, and took into account a number of alternative explanations.
One of the challenges in the US is that there are laws prohibiting researchers from testing cannabis, so we have to rely on product labels, which we know are unreliable,” said Meier.
What’s so remarkable here is that they can clearly demonstrate the loss of IQ points from childhood to age 45,” said Saxon.
“It is clear that in people using cannabis long-term, cognition is impaired.