Marijuana's recent legalization in two U.S. states means that its effects are bobbing into many people's conversations these days, not just High Times readers'. That makes its brain effects germane here at You, Illuminated. This is the last post in a three-part series. Parts 1 and 2 covered marijuana’s brain mechanism and seven short term effects. Now we’ll explore marijuana's long term effects on the brain. Even if you weren’t familiar with the brain mechanisms, the short term effects were probably familiar to most readers. The long-term effects, however, are probably less known and certainly more contentious.  

Some long-term effects are carry-over short-term effects. Since fat attracts THC and cellular walls in your body are composed of fat, THC gets sucked up by cells everywhere. THC then gets re-released gradually, which is why you can test positive for marijuana up to a month after you put down your hash pipe. So many short-term effects, like impaired memory and driving, might endure.

To test this idea, a recent study compared the short term memory and information processing speed of people who used marijuana in the past 4 weeks to people who had never used it. The recent marijuana users remembered less and processed information more slowly than the people who never smoked, even though the users didn’t smoke that day. Among the marijuana users, those who used more marijuana in the past 28 days performed worse than those who used less. The authors concluded, “cannabis use is associated with cognitive impairments that persist after abstinence.”

As long as THC remains in the body, it stands to reason, marijuana continues to affect the brain. What’s less clear is whether marijuana can actually change the brain. Once the last traces of THC have been excreted, could brain function remain altered for months, or years, after?

Few studies have addressed this question and the lack of clarity provides fodder for debate. To remove some ambiguity, Nora Volkow, director of the National Institute on Drug Abuse, recently wrote a summary in the New England Journal of Medicine of the scientific findings on the lasting effects of marijuana use. The entire article is worth reading, but I’ll highlight two key conclusions here.  

1. Risk of addiction

Smoking marijuana won’t set an irreversible course for addiction, but you must start using marijuana to become addicted. Almost one in ten people who use marijuana will eventually become dependent on it, according to findings from the National Epidemiologic Survey on Alcohol and Related Conditions. Half of the people who eventually met criteria for marijuana dependence did so within four years of their first use.

A little under half of people who seek treatment for marijuana dependence experience withdrawal when they try to quit, becoming anxious, irritable or depressed. These symptoms can precipitate relapse, making it harder to quit.

2. Alterations in brain development

The brain can be molded and developed throughout our lives, but since it is particularly malleable from the months before birth to our twenty-first birthday, researchers have wondered whether smoking marijuana during adolescence could change the brain’s development.

A team led by Andrew Zalesky from The University of Melbourne, Australia examined this question by performing MRI scans on regular marijuana users who started smoking in adolescence and a comparison group who never smoked. They found that the neural connections between the brain’s left and right hemispheres were impaired in the marijuana users relative to the non-users, which could mean that smokers’ brains have poorer internal communication.

Zalesky’s study, like much of the human work, compared people who already smoke to those who never smoked, so it doesn’t show that brains changed after the person started smoking. Smokers’ brains may have been different to begin with. The earlier someone started smoking, however, the more pronounced the impairments were, suggesting marijuana may cause changes.

Zalesky’s study is hardly unassailable proof, but the suggestion is powerful. Especially considering a study in New Zealand that followed over a thousand individuals from birth to age 38, showed that people who started smoking marijuana in their teens had lower IQs in their thirties compared to their childhood.

The IQ declines were not associated with personality, socioeconomic status, current drug use or presence of psychiatric diseases. The people who started smoking youngest and who smoked most heavily showed the greatest drop in cognitive performance.

The deficits extended beyond laboratory tests. People who lived with the study participants filled out questionnaires about the memory and smarts of participants. The participants who started smoking marijuana as teens had more memory troubles in daily life, according to the people who knew them best.

Causal studies of change due to marijuana are, of course, unethical in humans. The evidence we do have suggests that marijuana probably won’t scramble your brain, but if you start smoking heavily as a teen, it may slow your brain down.  

Clearly, we still have much to learn about the long-term effects of marijuana on the brain. The few studies available suggest that marijuana may change the brain’s function, especially if used when the brain is still undergoing rapid development. However, more studies need to confirm these suggestions and identify more specific alterations. Any conclusions may be premature, but it makes you wonder.

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