Study Links Long-Term Cannabis Use, Cognitive Deficits

In addition to the general increased use of marijuana due to its legalization, midlife adults are using cannabis at higher rates, said Madeline Meier, Ph.D., an associate professor in the Department of Psychology at Arizona State University and colleagues. Meier and her colleagues sought to better understand what these trends mean.

Meier and colleagues used data from Dunedin Longitudinal Study, a birth cohort of participants born between April 1972 and March 1973 in Dunedin, New Zealand. All participants were assessed regularly starting at age 3 until age 45. Starting at age 18, the participants were interviewed every three to six years about their substance use and past-year substance use dependencies, including their use of cannabis, tobacco, and alcohol.

Long-term cannabis users were defined as those who were dependent on cannabis or used cannabis weekly or more frequently in the past year at age 45 and also reported using weekly or more frequently at one or more previous assessments. Long-term tobacco users were defined as those who smoked daily at age 45 and reported smoking daily at one or more previous assessments. Long-term alcohol users were defined as those who drank weekly at age 45 and had a diagnosis of alcohol dependence at two or more assessments.

Cognitive tests were conducted at ages 7, 9, and 11 using the Wechsler Intelligence Scale for Children-Revised and again at age 45 using the Wechsler Adult Intelligence Scale-IV. Additional neuropsychological tests were administered to measure the participants’ verbal learning, attention, memory, processing speed, and more.

Of 938 participants who were assessed at age 45, 86 were considered long-term cannabis users. Relative to the normative IQ of 100, long-term cannabis users had average IQs as children (99.3) but below-average IQs as adults (93.8). Participants who never used cannabis had an increase in IQ of 0.7 points between childhood and adulthood. Long-term users, however, had a mean childhood-to-adulthood IQ decline of 5.5 points, which was significantly larger than the IQ decline of 1.5 points and 0.5 points among, respectively, long-term tobacco users and long-term alcohol users.

“That IQ change is important, especially as long-term cannabis users had average IQs in childhood,” Meier said. She and her colleagues also found that long-term cannabis users had poorer learning, memory, and processing speed than long-term tobacco or alcohol users. “If sustained, this study will continue to provide one of the most comprehensive investigations of the long-term effects of cannabis exposure on mental and brain health throughout the lifespan,” wrote Patricia Conrod, Ph.D., in a commentary published in the American Journal of Psychiatry in May. Conrod is a Tier 1 Canada Research Chair in Preventative Mental Health and Addiction at the CHU Sainte-Justine Mother and Child Hospital Research Centre in Montreal.

The study calls attention to the need for greater resources to determine if long-term cannabis users have an increased risk for dementia, Meier said. “We know from previous research that midlife cognitive functioning does predict a person’s risk for dementia,” she said. “Our cohort is only age 45 right now, and we don’t know what will happen to them in the next 20 or 30 years. But we do know that in the next 10 years, over 20% of the U.S. population will be considered older adults, and we have to start caring about the implications of such a huge percentage of the population aging and using cannabis at historically high rates.”

Das said Meier’s study adds to the available scientific evidence that long-term cannabis use is harmful. Moreover, she pointed out that the concentration of available cannabis is changing: Decades ago, the THC in cannabis products was typically around 20%. Now, some dispensaries sell cannabis products with THC levels as high as 80%.

“While we don’t have as much data on long-term use of these newer products, clinically and scientifically it would make sense that they are more harmful the more potent they are, though we cannot say anything with certainty yet,” Das said. “I just hope the implications of this study go beyond research and influence practice. More psychiatrists and physicians should screen for cannabis use because it is not benign [see article on facing page]. From effects on the individual to population-level effects on aging, work productivity, and health care, it is important we discuss this topic with our patients.” ■

“Long-Term Cannabis Use and Cognitive Reserves and Hippocampal Volume in Midlife”

“Cannabis and Brain Health: What Is Next for Developmental Cohort Studies?”