Judging by all the studies that have been conducted in hopes of understanding marijuana’s effect on the brain, the verdict is still out. While some evidence has shown long-term marijuana use can lead to decreased cognition, poor attention, loss of memory, and decreased IQ, others have shown that marijuana is much less damaging on the brain than alcohol.

A recent study conducted by King's College London and Sapienza University of Rome has found that so-called “skunk-like” cannabis can have a damaging effect on white matter in the brain. First, let’s go over what exactly “skunk-like” weed is.

“Skunk-like” cannabis refers to high potency marijuana that contains a larger amount of tetrahydrocannabinol (THC) compared to around a decade ago. In fact, “skunk-like” marijuana products high in THC are thought to be the most common form of cannabis in the United Kingdom.

“We found that frequent use of high potency cannabis significantly affects the structure of white matter fibers in the brain, whether you have psychosis or not,” said Dr. Paola Dazzan, reader in neurobiology of psychosis from the Institute of Psychiatry, Psychology & Neuroscience (IoPPN) at King's College London, in a statement. “This reflects a sliding scale where the more cannabis you smoke and the higher the potency, the worse the damage will be. There is an urgent need to educate health professionals, the public and policymakers about the risks involved with cannabis use.”

Dazzan and her colleagues used a Magnetic Resonance Imaging (NRI) technique known as Diffusion Tensor Imaging (DTI) to compare white matter in the brains of 56 patients who were treated for their first episode of psychosis at the South London and Maudsley NHS Foundation Trust (SLaM) to 43 healthy patients. They focused their attention on the corpus callosum — the largest white matter structure in the brain that is responsible for communication between the left and right hemispheres.

In addition to being the communication hub of the brain, the corpus callosum is also rich in cannabinoid receptors, making it a target for the THC content found in cannabis. Findings showed that frequent use of high potency marijuana was associated with a higher risk for mean-diffusivity — a marker for damage in white matter brain structures.

“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,” said Dr. Tiago Reis Marques, a senior research fellow from the IoPPN at King's College London. “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 on the type of damage these substances can do to the brain.”

While this new study blamed potency for marijuana’s debilitating effect on the brain, another recent study revealed that starting age and duration of use were the two most important factors when discussing pot’s impact on the brain. Researchers at the Center for BrainHealth at the University of Texas at Dallas found that structural wiring in the brain tends to decline after six to eight years of chronic marijuana use. However, marijuana users still display higher connectivity than non-users, even after this decline.

Source: Pariante M, Murray R, Dazzan P, et al. Effect of high-potency cannabis on corpus callosum microstructure. Psychological Medicine. 2015.