marijuana adolescents IQ

John D., a scientist of some renown was walking with his son, Jim, along a gurgling creek near their home. Jim was an adolescent turned off by everything the “real world” demanded of a youth of his age: focus, diligence, striving to excel in education. This was a drag for young Jim and a source of frustration for John.

The father-son conversation (more like a silent scream for the scientist/father and sullen resentment for the adolescent son) turned to the sore subject of a smoking pot. John was quoting chapter and verse of the latest research documenting the harm done to memory, focus, judgment, reasoning –all the cognitive functions pointing to a loss of neurons in the prefrontal cortex, the all-important executive center of the brain. Can’t his son see the life-long harm to his future success if he keeps on smoking pot? Doesn’t he care?

Jim, on the other hand, was citing “research,” published mainly in the counter-culture press, that said smoking pot in moderation is as harmless as drinking a couple of beers.


When it comes to the studies, who is right?

Who was right? In truth, John, a scientist trained in the scientific method, had a nagging doubt regarding the validity of the studies. What about all of those studies showing that pot smokers are deficient in cognitive functions. A closer look shows they only demonstrated associations, they did not prove causation. Was this population self-selecting? Could it be that mostly less-bright kids were drawn to smoking pot? This was not just any confounding factor; it was central to the premise that marijuana smoke contains neurotoxins.

That struggle between fathers and sons still continues several decades later, with the possible difference that many of today’s fathers are ex-smokers, and maybe even active smokers, today. The data that chronically smoking pot in the teenage years is associated with lowered IQ still holds. But a new study in PNAS (online edition, August 27, 2012)  avoid criticisms of the earlier work, which generally did not measure mental performance before marijuana use began. Its methods bring us closer to ascribing causation to the lowering of IQ among adolescent pot smokers.


What did the new study show?

The study drew on survey data from more than 1,000 people in New Zealand, everybody born in the town of Dunedin during a year-long span ending in 1973. In addition to IQ tests, they were interviewed five times between ages 18 and 38, including questions related to their marijuana use.

At age 18, 52 participants indicated they had become dependent on marijuana, meaning that they continued to use it despite its causing significant health, social or legal problems. Ninety-two others reported dependence starting at a later age.

Researchers compared their IQ scores at age 13 (pre-smoking) to the score at age 38 and found a drop only in those who had become dependent by 18. Those deemed dependent in three or more surveys had a drop averaging 8 points. For a person of average intelligence, an 8-point drop would mean ranking higher than only 29 percent of the population rather than 50 percent, the researcher said.

Among participants who’d been dependent at 18 and in at least one later survey, quitting didn’t remove the problem. IQ declines showed up even if they’d largely or entirely quit using pot at age 38, the analysis showed.

The researchers got similar overall results for IQ decline when they compared participants who reported having used marijuana at least once a week on average for the past year. The researchers had no data on how much was used on each occasion or how potent it was.

What’s the take-home lesson?

There are many lessons to be drawn from this excellent study. First and foremost, don’t tamper with a developing brain. The extensive and intricate brain circuits are in constant flux lasting from prenatal to age 18 or so. Neurons form contacts with neighboring ones to create circuits; circuits form connections with other circuits; functional centers form far-flung connections with other centers. Once these are formed a massive wave of neuronal death takes place so as to eliminate sources of “confusion” by superfluous neurons and increase the efficiency of the functional ones. The balance between emotional impulses (Kahnemann’s system 1) and the more thoughtful judgment of the prefrontal cortex and allied regions (Kahnemann’s system 2)  gradually takes shape, a process that reaches its conclusion at about age 18. Any interference, being genetic, chemical or psychological, may result in a less than optimal balance. This study is a perfect example.

The other lesson has to do with public health policy.

Pot is the most popular illegal drug in the world, with somewhere between 119 million and 224 million users between the ages of 15 and 64 as of 2010, the United Nations reported. Within the United States, 23 percent of high school students said they’d recently smoked marijuana, making it more popular than cigarettes, the federal government reported in June 2012.

Why did we have to wait so many years for such an important study? And why did we have to wait for New Zealand to conduct it? Unfortunately, most of our drug studies are funded by drug companies, and what pharmaceutical company has an economic interest in spending its money of such a study? The obvious answer then is that this is what NIH, the CDC and the FDA are for. But don’t hold your breath: all these agencies have been struggling to hold off the blunt ax of “small government” ideologues. The thin voice of the children’s constituency has no chance of being heard over the shouting.

Dov Michaeli, MD, PhD
Dov Michaeli, MD, PhD loves to write about the brain and human behavior as well as translate complicated basic science concepts into entertainment for the rest of us. He was a professor at the University of California San Francisco before leaving to enter the world of biotech. He served as the Chief Medical Officer of biotech companies, including Aphton Corporation. He also founded and served as the CEO of Madah Medica, an early stage biotech company developing products to improve post-surgical pain control. He is now retired and enjoys working out, following the stock market, travelling the world, and, of course, writing for TDWI.


  1. Dov,
    You have opened a very complex conversation. The results of the study you site are overwhelming. Without a doubt a developing brain should stay free of most extranious chemicals. Would that include sugar? Pharmaceuticals? Artificial coloring and sweeteners? Cigarettes? Alcohol? Coffee? The list goes on and on.

    The problem with this good information about cannabis is that it has been diluted by so much yellow press and misinformation about cannabis. In the 1930’s aand 1940’s the U.S. Government told us we would go crazy if we smoked “pot”. View the movie “Reefer Madness” sometime. In the 1950’s they told me in school I would end up using heroin if I smoked pot. The instructor even brought a joint into the classroom so we would know what it looked like. In the 1960’s and 1970’s we were told we would lose all our motivation and ambition if we smoke marijuana. Tell Ted Turner that. He smokes a joint a day. And then there was Nancy Reagan and “just say no”.

    The bull shit goes on and on. If you want to view a very interesting movie on the government, marijuana and craziness get “AKA Tommy Chang”.

    There are over 200 medical conditions that respond favorably to cannabis. I find it quite helpful for back pain, hemorrhoids, insomnia, mild depression, anxiety, and mild body pain. View a film from Canada titled “The Union: The business behind getting high”.

    Thanks for the blog Dov.


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