Does Cannabis Decrease IQ?

The harbor and hills around Dunedin, New Zealand, are the remnants of a volcano, but an eruption of a different sort came out of this city of 120,000 in 2012. The results of a 25-year study of 1,037 Dunedin citizens born in 1972/1973 were published that year.¹

Madeline Meier analyzed data gathered by the Dunedin Multidisciplinary Health and Development Research Unit, which sought to determine whether the cognitive decline found in long-term cannabis users is concentrated among adolescent-onset users. Cannabis use was recorded at ages 18, 21, 26, 32, and 38. A battery of neuropsychological tests was conducted at age 13, before the onset of cannabis use, and again at age 38.

Unfortunately, the results of the Dunedin study have been widely misinterpreted. Careful reading is needed to understand exactly what the data prove. Even the U.S. National Institute on Drug Abuse did not clarify the results well on its website. As a result, the press and too many people believe that the more persistently adults use marijuana, the more their IQ decreases. The article actually concluded [emphasis added], “In fact, adult-onset cannabis users do not appear to experience IQ decline as a function of persistent cannabis use.”

The major findings in Meier’s analysis of the Dunedin data include the following:

• The only individuals at age 38 who showed a reduction in IQ were those who were using marijuana at least once a week during adolescence.
• Individuals who began marijuana use as adolescents are more likely to have persistent use as adults.
• The more persistently an adolescent-onset user continues to use marijuana during adulthood, the greater their reduction in IQ is at 38.
• Those who began marijuana use the earliest and continued throughout adulthood lose an average of eight IQ points, which is sufficient to alter academic success and job opportunities.
• Stopping marijuana use did not fully restore neuropsychological functioning among adolescent-onset, former persistent cannabis users. Indeed, among adolescent-onset, former persistent cannabis users, impairment was still evident for one year or more after the cessation of use.

Tests measuring IQ are comprised of an aggregate of several subtests. The Dunedin study reveals that impairments in early-onset persistent users are detected across five areas: Executive Functioning, Processing Speed, Memory, Perceptual Reasoning, and Verbal Comprehension, with the most robust findings being decreased processing speed and impaired executive functions. More specifically, digit symbol testing reveals reduced processing speed, Wisconsin Card Sort reveals reduced mental flexibility and increased repetition of ineffective strategies, and Stroop testing reveals increased impulsivity and difficulty ignoring distractions.

Participants in the study also identified informants who “knew them well.” These informants reported significantly more everyday life problems with attention and memory among individuals with the most persistent cannabis dependence. This confirms that cognitive deficits caused by cannabis are of more than academic interest.

Adolescents are at high risk of being harmed by cannabis because the structure of their brains is still developing. The onset of puberty introduces a period of rapid brain development. An explosion of new connections (synapses) between brain cells occurs like the wild growth of twigs in a young tree each spring. Gardeners routinely prune twigs that cross and cover established branches to increase the tree’s health. Similarly, learning and experience strengthen some of the brain’s new synapses, while others are pruned away by disuse over the next decade and a half.

The frontal lobes are the last area of brain structure to mature. Persistent use of cannabis reduces the number of natural cannabinoid receptors (CB1R) in the frontal lobes by 20 percent.² Multiple studies show that adolescents using enough cannabis to reduce CB1R in the frontal lobes have impaired executive functions that last longer after stopping cannabis use than for adults.^3,4,5 Impaired executive functions mean a person has difficulty with abstract thinking, sequencing, planning, avoiding distractions, and shifting set (i.e., seeing things in a new light and changing your mind), to name but a few higher cognitive functions.

Presenting this data is not intended to demonize cannabis, but rather to establish the inconvenient truth that frequent use is riskier for adolescents than for adults. The Dunedin study also documented that individuals with early-onset cannabis use are more likely to be persistent users during adulthood. This finding is consistent with data showing that Cannabis Use Disorder (CUD) occurs more frequently and more quickly in early-onset users.⁶

For example, an 11-year-old who starts using cannabis has over a 17 percent chance of being addicted by age 13. Starting at 12 has over a 16 percent chance of addiction by age 14. Starting at 16 runs almost a 13 percent risk of addiction by 18. Waiting until 18 to start cannabis lowers the risk of addiction by age 20 to about 7 percent. And waiting until 20 brings the risk of developing addiction within two years down to 3 percent at age 22. The data clearly establish that early onset of marijuana use increases the rate and speed of addiction.

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Comprehending that one’s cannabis use is causing more problems than it is worth and changing behavior is more difficult when the frontal lobes are functioning at less than an optimal level.

This post was excerpted from From Bud to Brain: A Psychiatrist's View of Marijuana (Cambridge University Press, 2020).