The truth about cannabis and your white matter

An animation of the human body holding a dirt with a cannabis plant

Recently, a study published by King’s College, London caused a media frenzy as it associated heavy use of potent cannabis with structural changes to the brain, most notably a decline in the quantity and integrity of the white matter.

As is so often the case with such studies, certain media outlets immediately leapt to wild assumptions that cannabis was to blame for this perceived difference in brain structure, and that policy changes needed to be immediately made to reflect this. But is this truly the case?

Again, as so often occurs with these studies, when we look a little deeper into what is said in the text, we find that there is a lot more to the story than these simplistic media articles would have you believe.

What was this study about, exactly?

Before we wade into the various faults of the study, let’s take a quick look at exactly what it says. The paper took a sample of 99 people, 56 of whom had reported a prior psychotic episode, and assessed their brain matter using MRI scans of a specific type known as Diffusion Tensor Imaging (DTI). This advanced technique measures in 3D the rate of diffusion of water molecules through tissues of the body, and allows users to analyze specific tissues and structures with astonishing precision and detail.

The scans assessed white matter, which is made up of densely-packed bundles of myelinated axons (nerve fibres) and connects different areas of the grey matter together. The grey matter houses the neurons themselves—one way to look at it is the neurons generate the information and the axons carry it to new neurons, which then act on it. The study particularly investigated the corpus callosum, which connects the left and right hemispheres and is the largest white matter structure in the brain.

What did the study find?

DTI imaging of the corpus collosum
The corpus collosum, the largest region of white matter in the brain, as depicted by DTI imaging (© Andras Jakab)

An important measurement with DTI scanning is mean-diffusivity (MD). This measurement is one way of determining the rate at which water molecules diffuse through tissue, and when MD is higher than normal, it is thought to be associated with reduced density of axon bundles, and damage to the myelin sheaths that surround and insulate each axon and allow signals to rapidly pass along their length.

By measuring the corpus callosum, the researchers found that the heaviest cannabis users exhibited the greatest increase in MD compared to non-cannabis-using controls. This association was seen across the board in heavy cannabis users, even those that had not experienced psychosis.

One of the lead researchers, Dr Tiago Reis Marques stated, “‘This white matter damage was significantly greater among heavy users of high potency cannabis than in occasional or low potency users, and was also independent of the presence of a psychotic disorder”.

Another head researcher, Paola Dazzan added, “There is an urgent need to educate health professionals, the public and policymakers about the risks involved with cannabis use”.

What’s wrong with the study?

So what’s wrong with the picture? Well, first off, it seems that both the media and the researchers have committed the cardinal sin of conflating correlation with causation. Yes, there is an association between the rate of potency of cannabis use and with these structural differences, but there is nothing in the study to suggest that cannabis caused these changes.

It could prove to be the case that pre-existing structural abnormalities in certain individuals predispose them to heavy cannabis use, rather than heavy cannabis use causing the changes in and of itself.

Furthermore, the potency of the cannabis that the study subjects self-reported as using was categorized simply as low potency (hash-like) or high-potency (skunk-like); without making a better effort to determine cannabinoid levels and ratios, this appears to be somewhat of a useless designation. The study also did not determine how much cannabis was consumed by each subject, for how many years, or whether other substances were used alongside it.

As well as this, some have mentioned the small size of the sample and the difficulty of extrapolating meaningful results from this. However, the sample size is not exceptionally small for a preclinical study such as this. What is certainly true is that firm conclusions in media reports have been drawn from inconclusive data (along with misleading statements from researchers that should know better) that by its very nature necessitates the need for further research.

What would a study that actually proved a link look like?

A persons’ brains were analysed using DTI
Subjects’ brains were analyzed using DTI, a form of MRI scanning (© Muffet).

The paper in question is a cross-sectional study, which looks at different sample groups at a given point in time and compares certain variables to identify possible associations. By their very nature, cross-sectional studies are unable to determine cause and effect, as they do not look at time trends. In this case, the researchers cannot say if the changes were already there, so they cannot state they occur as a result of cannabis use, however strongly they may feel it implies it.

Cross-sectional studies are also inherently limited in that they directly compare a restricted set of variables from which to draw conclusions, and do not control for other confounding factors. In this case, the perceived damage could be caused by something entirely other than cannabis use, which the researchers simply did not account for, such as alcohol or tobacco use.

In order to prove that cannabis use causes structural changes to the brain in humans, it would be theoretically possible to conduct a randomized controlled trial in which cannabis would be administered to previously-inexperienced subjects and any rate of change observed.

However, for ethical and legal reasons, this would be impracticable, as if brain damage were to result, those previously healthy subjects would obviously be very displeased, to say the least, and they and their families would no doubt be looking into legal action! Of course, in the UK, where this study was conducted, cannabis remains illegal save for a tiny list of approved conditions, so there’s another reason that this type of research can’t happen. It can be conducted on animals, but that’s of limited usefulness when looking for answers specific to humans.

Thus, researchers must content themselves with looking at people who voluntarily consume cannabis and are prepared to be studied. So, the best choice when attempting to establish cause and effect is to use a longitudinal cohort study, which follows the same group of individuals over time and tracks changes in consumption habits (as well as other lifestyle factors) and compares them to observed physical changes.

There have been a handful of longitudinal studies investigating cannabis and mental health, but apparently none that have specifically investigated cannabis’ effect on the white matter (save for one that looked at teens who used both cannabis and alcohol, and which implied that alcohol was particularly likely to be responsible for white matter degradation).

Some media backtracking…

Closeup of the potent cannabis plant
Heavy use of potent cannabis was associated with white matter damage, but no causative link was established (© DonGoofy)

It is interesting to note that the press release on the King’s College website originally was entitled “Study shows white matter damage caused by “skunk-like” cannabis”; this has now been amended to “Study shows white matter damage may be caused by “skunk-like” cannabis” (emphasis added).

Of course, as is usually the case with such studies, the more moderate media outlets (The Guardian, The Independent) have made sure to include the “may” or “can”, while the more strident “broadsheets” (The Telegraph) have made sure to omit it.

Of course, the gutter right-wing press pulled no punches, with The Sun memorably proclaiming that “Scientists warn smoking ‘skunk’ cannabis wrecks brains” and The Mail trumpeting “Proof strong cannabis DOES harm your brain”; this egregious distortion of the facts was enough to motivate the UK’s National Health Service to publish an article debunking the sensationalist reports.

However, despite the flaws in the study and the media reporting on it, it does at least indicate the possibility that cannabis could, in fact, be what’s causing the change, and thus its findings should not be dismissed out of hand. To do so would be to fall into the same trap of assuming the question is settled, when in fact it has only just begun to be answered.

  • Disclaimer:
    This article is not a substitute for professional medical advice, diagnosis, or treatment. Always consult with your doctor or other licensed medical professional. Do not delay seeking medical advice or disregard medical advice due to something you have read on this website.


13 thoughts on “The truth about cannabis and your white matter”

  1. I am actually looking to see if CBD is useful in helping with White Matter Disease. My grandmother has been diagnosed since 2014 and it has progressed exponentially to dementia like symptoms I won’t go into detail about. I read a few pubmed articles that show CBD does do have positive effects in mice with white matter disease, but cannot find any published articles regarding this in humans. I am going to try it anyway with my grandmother as I know CBD is safe and something is better than nothing at this point and I cannot stand seeing her progression with this.

  2. So this information shows for the most part that cannabis is not harmful to your brain but most people smoke it, is that what? What about the harm it does to your lungs? I’m 70 years old, I’m a 60s chick and I’ve had friend die with lung disease from the use of marijuana. PLEASE understand I’m not judging this, I’m just would like to know if there is a study on the effects on the lungs.

    1. Scarlet Palmer - Sensi Seeds

      Hi CCR,

      Thanks for your comment. I’m sorry to hear about your friend. This article, on cannabis and lung health, discusses the results of a twenty-year study into the effects of cannabis smoke on human lungs. I hope this information is useful to you, and that you continue to enjoy our blog.

      With best wishes,


  3. Moderation , as in most things, is the key.
    I think that if you treat this plant with the respect it deserves and use it when you really feel you need it etc. – the results will be more positive- on a physical [brain] level too.

  4. i had my mri taken and the prognosis was ‘decreased white matter because of substance abuse’. could someone take a look at it and analyze it?

    1. Scarlet Palmer - Sensi Seeds

      Hi Alok,

      I am going to publish your comment as I see no reason not to, and I understand that you are trying to find more information about your MRI. However, I want to just remind you that this is a public forum, and unless someone is a qualified medical practitioner specified in the field you are asking about, the replies are going to be based on anecdotal evidence and experience. For an expert analysis, please consult an expert – maybe someone at the place where you had the MRI done, or your GP, can explain it to you?

      With best wishes,


  5. I would like to see their brain imaging results of heavy “skunk” (lol) daily users, compared against those of Alcoholics. Perhaps a more startling headline would be “Scientists prove Alcohol wrecks brains”

  6. Did the brain and the person function better? The “new” shape is maybe the way how a brain has to look.

  7. Michael Vipperman

    The actual data in the study shows a conspicuous absence of any credible harm. A very small difference in a single brain area is not credibly “damage.” Even if marijuana was actually causing this effect, which it’s extremely likely that it was not, the study would constitute proof that marijuana is not harmful to the brain, because if it were harmful then we should expect a much larger difference, and probably in more than one region. Indeed, no brain change has been consistently associated with marijuana use. Some studies say larger amygdala, others say smaller, averages out to nothing. Some studies found smaller hippocampus, others found no hippocampal differences. Some studies found white matter differences, others found no white matter differences. The studies which found harm were all poorly designed and ineptly controlled: we know that childhood trauma causes much more severe damage in all the same brain areas being studied, and we know that trauma survivors are vastly more likely to become dependent on marijuana, but these evil frauds claim, without controlling for trauma, that trauma-related pathology is caused by marijuana, even though it’s not found in the majority of studies.

    Furthermore, these inexcusable hacks have repeatedly refused to acknowledge the benefits their subjects have told them they are receiving and the relevance of those effects to brain health. They conclude that “self medication” can be rejected on the grounds that THC does not reduce effectively treat positive symptoms of psychosis, even though their subjects told them that they use it to alleviate negative symptoms and not positive symptoms.

    There are many questions which still require answers, but the position they’re advancing is empirically unsupported and utterly indefensible for reasons which go far beyond correlation versus causation. We need to talk less about certainty and more about effect size; when we see studies presenting the “proof of harm,” and what they show us is this insubstantial, an appropriate responses is “what, that little?” — but an even more appropriate response might well be rage at this level of dishonesty from a prestigious university. Serious investigation of possible harms is absolutely necessary, but this paper was fraud, and the entire faculty of Psychosis Studies at King’s College London should be defunded, if not pilloried.

      1. Pot industry sounds exactly like the tobacco industry did in its prime.

    1. Pretty good read. Its Mercury Fillings that’s deteriorating my Myelin Sheath!!!!! Thats what I’m Screaming !!!!!!

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    The Sensi Seeds Editorial team has been built throughout our more than 30 years of existence. Our writers and editors include botanists, medical and legal experts as well as renown activists the world over including Lester Grinspoon, Micha Knodt, Robert Connell Clarke, Maurice Veldman, Sebastian Maríncolo, James Burton and Seshata.
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