Strong weed can damage your brain


Bad news for (some) cannabis smokers, a new study on the effects of weed has shown that smoking the stronger strains can lead to nerve damage and psychosis. This study is thought to be the first to look at the effects of potency of drugs on brain structure. Scans were taken of the brains of people who regularly smoke potent skunk-like cannabis and revealed differences in the white matter that connects the right and left sides of the brain. The white matter also transmits messages between the left and right hemispheres, thus the damage caused by smoking strong cannabis impairs communication efficiency between the hemispheres. The changes were not found in scans of people who only smoke the less potent forms or in those who never smoke cannabis at all.

Of the people who took part in the study, just over half (56) were patients who had suffered a first episode of psychosis, the remaining 43 were healthy volunteers from the local community. The scans showed a 2 per cent increase in “mean diffusivity” in the corpus callosum (the broad band of nerve fibres that connect the two hemispheres of the brain). Paola Dazzan, a neurobiologist at the Institute of Psychiatry at King’s College London, confirms that this “reflects a problem in the white matter that ultimately makes it less efficient, it suggests there is a less efficient transfer of information.” Dazzan says the damaging effects appear to be related to the active ingredient tetrahydrocannabinol (THC) in cannabis. The weaker forms contain approximately two to four per cent THC, whereas the stronger strains (of which there are over 100) contain 10 to 14 per cent THC, according to DrugScope.

However, Dazzan notes that the study cannot confirm that high levels of THC in cannabis cause the damage to white matter as this may be the difference in people’s brains that cause them to smoke skunk in the first place. “It is possible that these people already have a different brain and are therefore more likely to use cannabis. But what we can say is if it’s high potency, and if you smoke frequently, your brain is different from the brain of someone who smokes normal cannabis, and from someone who doesn’t smoke cannabis at all,” she clarifies.

Despite the lack of definitive answers, Dazzan has still urged those who use cannabis and those who work in public health to alter the way in which they perceive the use of cannabis. “When it comes to alcohol, we are used to thinking about how much people drink, and whether they are drinking wine, beer, or whisky. We should think of cannabis in a similar way,” she said. “As we have suggested previously, when assessing cannabis use, it is extremely important to gather information on how often and what type of cannabis is being used. These details can help quantify the risk of mental health problems and increase awareness of the type of damage these substances can do to the brain.”

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