"Scientists warn smoking 'skunk' cannabis wrecks brains," The Sun reports, somewhat simplistically. A small study found some users of the high-strength skunk strain of cannabis had changes in nerve fibres in a specific part of the brain.

Researchers used MRI scanners to scan the brains of 99 adults – some with psychosis, some without – looking for any links between small changes in their brain structure and their cannabis habits.

The researchers looked specifically at the effect on the fine structure of the corpus callosum. This is a band of nerve fibres joining the left and right sides of the brain and is thought to help different parts of the brain "communicate" with each other.

They found users of skunk – as well as those who used any type of cannabis on a daily basis – had different structural changes in the corpus callosum, compared with those who smoked less or lower-strength strains.

What this study doesn't tell us is whether these structural changes do any harm or cause any negative mental health effects – which is why The Sun's headline is too strong. The study simply didn't look at this.

The effects of cannabis use – both in the short and longer term – are not firmly established. But cannabis is known to be one of many substances that can trigger a psychotic episode. Read more about psychosis.

The study adds new knowledge about the potential effect of cannabis smoking on the brain, which other researchers can build on. But this was exploratory research and cannot provide any concrete cause and effect conclusions.

Where did the story come from?

The study was carried out by researchers from King's College London and the Sapienza University of Rome.

It was funded by a King's College London Translational Research Grant, the National Institute for Health Research (NIHR) Mental Health Biomedical Research Centre at the South London and Maudsley NHS Foundation Trust, and King's College London.

The study was published in the peer-reviewed Psychological Medicine on an open-access basis and can be read online for free.

Generally, the UK media covered the story accurately, but some of the headline writers overstepped the mark. The Sun's headline, "Scientists warn smoking 'skunk' cannabis wrecks brains", and the Daily Mail's, "Proof strong cannabis does harm your brain", were not based on any evidence.

This type of study cannot prove cause and effect, only suggest a possible link, so "proof" is too strong a term. Also, the study didn't look at how the small changes in the brain associated with skunk affected thoughts or other brain functioning, so it was not fair to say skunk "wrecks" the brain.

This study wasn't designed to look at the effect of skunk on mental health illnesses, only small changes in brain structure, so it tells us little about the link between cannabis use and the development of a mental health illness.

What kind of research was this?

This cross-sectional study looked for differences in the structure of a specific area of the brain called the corpus callosum in people with psychosis and those without.

It also looked at how this was linked with their reported cannabis use. The researchers were most interested in the effect of cannabis potency and how regularly cannabis was used.

The research team says high-strength cannabis (skunk) has been associated with a greater risk and the earlier onset of psychosis – the experience of hallucinations or delusions, a characteristic feature of the mental health condition schizophrenia.

However, the possible effect of cannabis potency on brain structure has never been explored. The researchers set out to investigate this by studying the fine structure of the corpus callosum, a band of nerve fibres joining the left and right sides of the brain.

This type of study can't prove cannabis causes changes in brain structure or any associated mental health illness. A long-term cohort study would be needed for this – a randomised controlled trial wouldn't be appropriate for ethical and, in the UK, legal reasons. But this type of study can point to possible or probable links for further investigation, a useful exercise to guide the next round of studies.

What did the research involve?

A group of 56 people with psychosis (37 cannabis users) and 43 people without psychosis (22 cannabis users) had their brains scanned. The scans were used to look for possible links between their cannabis habits and any differences in the structure of the corpus callosum area of their brains.

Those with psychosis had been medically diagnosed with first episode psychosis, which simply means someone who has experienced psychosis for the first time. Most of those with psychosis were taking antipsychotic medication (53 of 56), just three were not.

The brain scans used an MRI imaging technique – diffusion tensor imaging tractography – that maps how different parts of the brain are linked to each other and how easily information is transferred between both sides. This technique measures the efficiency by which signals in the brain travel (diffusivity), where low diffusivity scores indicate a healthy functioning brain and high diffusivity may indicate some form of damage.

The team looked at four common diffusion tensor imaging measures:

fractional anisotropy (FA)

mean diffusivity (MD)

axial diffusivity (AD)

radial diffusivity (RD)

FA is a sensitive way of picking up small brain structural changes and is relatively generic. MD, AD, and RD give more specific indications of where changes happen.

All participants filled in an illicit drugs questionnaire that included their cannabis smoking habits, when they first started, the strength they used, and how often they used it.

The statistical analysis took account of the following confounders:

sociodemographic factors

age

gender

ethnicity

some lifestyle factors, such as alcohol intake

What were the basic results?

There were some interesting results, not all of which were picked up in the media reports. For example, those diagnosed with psychosis were more likely to have used cannabis at some stage in the past compared with those without psychosis.

But there were no differences between people with and without psychosis in terms of how long they had used cannabis, how old they were when they first used the drug, the type of cannabis used, how often it was used, and the strength.

Three of the four measures of corpus callosum function were no different in people with psychosis compared with those without (MD, RD, AD). FA was found to be different, but was borderline statistically significant, meaning there is a reasonable probability the result is down to chance – specifically, a 1 in 25 chance, p=0.04.

As the corpus callosum structure wasn't that different between those with and without psychosis, the researchers pooled the groups to study the effect of cannabis on the brain. Overall, they found the corpus callosum structure was negatively affected in people using high-potency cannabis, compared with those using a lower-potency strain or not using cannabis at all, across MD, AD and RD diffusion measures, but not the more generic FA.

These alterations were similar in users with and without psychosis. A similar mixed pattern was found for frequency of use, with daily users having the most changes compared with occasional or never users. No link was found between those first using cannabis before the age of 15 and those starting after in terms of changes in the corpus callosum structure.

How did the researchers interpret the results?

The researchers concluded: "Frequent use of high-potency cannabis is associated with disturbed callosal microstructural organisation in individuals with and without psychosis.

"Since high-potency preparations are now replacing traditional herbal drugs in many European countries, raising awareness about the risks of high-potency cannabis is crucial."

Conclusion

This research studied the brains of 99 people – some with psychosis and some without – looking for any links between small changes in their brain structure and their cannabis habits. The researchers looked specifically at the effect on the fine structure of the corpus callosum, a band of nerve fibres joining the left and right sides of the brain.

They found the corpus callosum wasn't very different in those with or without psychosis. But smoking high-strength cannabis (skunk) and using any type of cannabis daily was linked to structural changes in the corpus callosum, compared with those who smoked less or lower-strength cannabis.

What this study doesn't tell us is whether these structural changes do any harm or cause any negative mental health effects. The study simply didn't look at this, a subtlety much of the news reporting failed to recognise.

The study also can't tell us whether cannabis use is the direct cause of these observed differences, or whether other factors could be having an influence. Cohort studies following people over time, examining their cannabis use and carrying out follow-up brain scans, would be beneficial to look at this.

The researchers made the best of what they had in terms of collecting a sample of almost 100 people and analysing the results appropriately.

However, as with all research, this study has its limitations. For example, 100 people isn't enough if you are splitting people into many groups, such as those with and without psychosis and different levels of cannabis use.

Some of the group numbers start to become quite small, which increases the chances you won't have enough people to find statistically significant differences, even if they exist. It can also throw up some unusual findings that wouldn't be the case in a larger group. This study carries these risks.

Similarly, the findings themselves weren't entirely consistent. There is a mix of significant and non-significant findings for the four measures taken (FA, MD, RD and AD). This lack of consistency muddies the picture somewhat and reduces our confidence in the findings a little.

The effects of cannabis use – both in the short and long term – are not firmly established. This study adds new knowledge about the potential effect of cannabis smoking on the brain that other researchers can build on. But it was exploratory research and so cannot provide concrete cause and effect conclusions.

Cannabis is a class B drug that is illegal to possess (up to five years in prison) or supply (up to 14 years in prison). And while it may not trigger mental health problems in everyone, it may make pre-existing symptoms such as depression and paranoia more severe. If you think you may be using cannabis to cope with mental health problems, contact your GP for advice.

Analysis by Bazian

Edited by NHS Website