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Does Marijuana Cause Brain Damage?

By on November 2, 2012

Summary (click to view)


• A common belief is that marijuana use will lead to brain damage
• This myth was popularized in the 70s by a study conducted on monkeys
Later studies revealed flaws in the original study and found no evidence of brain damage
Studies conducted on humans have come to similar conclusions (although some evidence is conflicting)
Studies even show that cannabinoids can promote the growth of brain cells
• Although research shows that marijuana use does not cause brain damage, abuse of substances such as alcohol and caffeine can

TruthOnPot.com – Many people still believe that marijuana kills brain cells and that chronic use will eventually lead to brain damage.

This frightening belief was popularized in the 1970s when Ronald Reagan famously stated, “the most reliable scientific sources say permanent brain damage is one of the inevitable results of the use of marijuana.”

In the following years, Reagan’s scientific sources would be discredited and subsequent studies would be unable to replicate any evidence of brain damage, yet the myth remains as prominent today as it was 40 years ago.

False Monkey Study

Dr. Robert G. Heath was a researcher at Tulane University in New Orleans when he reported the findings of an experiment that apparently proved a connection between marijuana use and brain damage. Dr. Heath had conducted the study on rhesus monkeys by exposing them to an equivalent of 30 marijuana joints per day. After 90 days, the monkeys began to waste and die. When they were later autopsied, Dr. Heath reported significant brain damage in the monkeys that had been exposed to cannabis.

Despite strong support from the federal government, the study was heavily criticized for inaccurate procedures upon its release. Critics suggested that suffocation may have been the actual cause of brain damage instead of marijuana itself.

The findings were challenged and ultimately dismissed by a pair of larger, better-controlled studies – one by Dr. William Slikker of the National Center for Toxicological Research and the other by Charles Rebert and Gordon Pryor of SRI International – that attempted to replicate Dr. Heath’s results without success. The studies showed no change in the brain structure of monkeys that were given daily doses of marijuana for up to one year.

Human Studies

A number of studies have been conducted on human marijuana users over the past few decades, beginning in the 1970s as well.

In 1971, a study published by Campbell et al. involving 10 heavy users seemed to demonstrate a decrease in brain cells in a specific area of the brain, adding further to public concerns over marijuana use and brain damage. However, subsequent studies (Co et al., 1977, Hannerz and Hindmarsh, 1983 and Kuehnle et al., 1977) failed to replicate these findings, suggesting that the initial study was flawed yet again.

Recent research conducted on human subjects has also revealed conflicting evidence on marijuana use and brain damage. A study published in 2000 by researchers at the University of Iowa found no evidence of cerebral atrophy or regional volume differences in MRI scans of frequent cannabis users when compared to non-users.

On the other hand, the support for marijuana use causing brain damage seems to be stronger in studies that separate users by age. An early study conducted by Wilson et al. (2000) found that marijuana users who started using before the age of 17 seemed to exhibit noticeable changes in brain matter volume. However, the study found no differences in whole brain volumes overall.

In 2012, Duke University researchers published the results of a study that seemed to show that adolescents who started using marijuana before the age of 18 – and diagnosed as addicted by the time they were 38 – experienced a drop in IQ by early adulthood. The study suggested that heavy users could lose as much as 8 IQ points. However, an analysis published a few months later in the same medical journal determined that the findings of the previous study were once again plagued with inaccuracies and – using a statistical model to evaluate the original study’s data – seemed to show that there was no correlation between marijuana use and a drop in IQ.

Finally, studies that have investigated the effects of marijuana on brain cells have revealed findings that directly contrast with the popular belief of marijuana causing brain damage. In fact, a study published in 2005 showed that a synthetic form of THC could increase the growth of new brain cells – a process known as neurogenesis. CBD has also been found to have a similar effect on brain cell generation. Taken together, these findings seem to contradict the notion that marijuana may cause abnormal brain development in adolescent users.

Warning: Alcohol and Caffeine Can!

Most commonly abused substances – including alcohol and caffeine – can cause permanent brain damage so it may be a surprise to hear that marijuana is safe in this regard. Although users of marijuana may not be at risk of neurological damage, cannabis is still a psychoactive substance that subjects its user to a temporarily altered state of mind.

  • B. Nicholes

    hmm, nice findings…nice to know I didn’t necessarily smoke myself silly

  • anon

    Finally a site that includes the counter analysis of that obviously flawed study. And this new study suggesting abnormal brain development in teens is not convincing at all, despite all those ridiculous news sites shot gunning these findings all over the web. Clearly just trying to scare the public, even though none of these findings is factual and it is still unconfirmed.

    • dan690

      You are just looking for an excuse.

      • Darius

        And you spread your claims without reference.

        • dan690

          You don’t need references for the obvious but if you google it you will find all the references that you don’t really want.

          • Darius

            If some one wrote claims muts support them. In ohter case he is simply quack. Please show me your reference because for now there is no evidence that cannabis damage brain like there is no evidence they dont.

          • dan690

            You must have taken another hit.

          • Darius

            Because I get some studies, I must wait for approval so please reply to next comment not this.

          • RJohnstonAZ

            Some do no research to make claims, Harvard using MRI’s has detected changes do occur in the brain of marijuana smokers, especially in young adults.

          • Darius

            No they found only that this people have changes in brain, but you dont know what came first. Maybe this changes cause people to start smoking in the first place. Second can be completly unrelated. Remember correlation dont imply causation. This is main flaw of epidemiological studies.

          • caem

            You will also find many references by studies that show no link between marijuana usage and brain damage (eg. University of Colorado study published on 28th Jan., 2015 in the Journal of Neuroscience) so you’re either reading selectively, taking in only what you want to believe or haven’t actually done any research.

          • dan690

            When you have two completely opposite results from the same study you agree with the one that makes sense. When you are around people from this category only one makes sense.

          • RJohnstonAZ

            Harvard using MRI’s have detected changes do occur in the brain of marijuana smokers.

  • anon_

    Its nice to see that you’ve included the counter analysis of that obviously flawed study stating that cannabis may reduce IQ points of adolescents. Most of these anti drug studies ignore environmental and economic factors, which are very difficult to filter out. And this new study stating cannabis may alter adolescents brain is not very convincing, since they still have no real link or factual information confirming this. But they’re using it as a scare tactic none the less, several news sites have begun shot gunning this bull**** all over the web. All except huffington post, which is known to report only facts and significant info. At least they included the counter analysis as well.

    • RJohnstonAZ

      Notice they do not include studies being done at Harvard using MRI’s where they have detected changes do occur in the brain of marijuana smokers. Those who are in favor tend to ignore reliable research which proves what they wish to ignore.

  • dan690

    You only have to know a pot smoker to know their brain cells are being killed off.

    • Darius

      You like author of this article spread quackery with no proof. Show me your reference, because as neurology student Im intersting with this topic and dont see any proof, except when incompetent peope spread them.

      • dan690

        You must be blind and should look for another line of work.

        • Darius

          Read my second answer to you.

    • caem

      There are quite a few things that cause the destruction of brain cells, the aging process being one, so you must be very familiar with the medical history of these pot smokers to know that this is the cause. You must also know a significant amount of them because knowing only a couple is far from a significant enough number to be able to claim that something that is a fact and would apply to a whole population.

      I’m assuming you equally as opinionated that alcohol should be made illegal due to the destruction it causes to brain cells ? I’d expect you would actually be more vocal because the literature on the link between alcohol abuse and brain cell destruction is much more conclusive than with marijuana – I couldn’t find any studies that found chronic alcohol use had no effect on brain cells.

      I do not know your views on alcohol but if, like most, you accept it and do not think it should be made illegal, while maintaining marijuana should be, I fail to see any logic at all with this thinking and find them quite hypocritical.

      • dan690

        I see you are having a little trouble with the reasoning part also. I never mentioned alcohol and never mentioned legality of either. You are so fixed on making the results go your way that you don’t bother with facts. Keep smoking, the results will only get worse.

  • Darius

    “Most commonly abused substances – including alcohol and caffeine – can cause permanent brain damage so it may be a surprise to hear that marijuana is safe in this regard” The biggest quackery I read today show proof that caffeine damage brain. For now there is no evidence to show that caffeine causes or leads to brain damage. You know why people think that cannabis damage brain because people like you. Wroting B-S whithout any proof.

  • ColinA

    Dan690= Ignorance at its finest.

  • B.nunez

    marijuana doesn’t make you stupid you were just stupid to begin with

  • RJohnstonAZ

    You can prove anything with statistics, as long as you use the stats that prove what you want to prove. There are even ways to prove we do not exist, etc… Too many “studies” are based on a very small proportion of the population. Many have been found to use as few as 50-100 when to be accurate well over 1000 or even 10,000 would get more accurate results. The Russians have been doing research with people from birth to death, including their entire medical history in research. One of the research programs involves cancer, and there are many others. But don’t know of any involving marijuana, though would not be surprised if they are doing such research.