The study, published in the journal Psychological Medicine, found that “skunk” cannabis damages the corpus callosum, a bundle of neural fibers that allows communication between the brain’s left and right hemispheres.
The two sides of the brain have very different functions. For instance the right hemisphere controls the left side of the body and the left hemisphere controls the right side of the body. While the Bible says, "do not let the left hand know what the right hand is doing", it is speaking metaphorically. In reality, coordination between the two sides of the body is highly recommended. And there are many other reasons the two sides of the brain should be communicating.
In Holland, where marijuana is legally sold in cafes, skunk has been reclassified as a hard drug and is therefore illegal.
The illegal drug contains high levels of tetrahydrocannabinol (THC), the chemical responsible for the high users receive.
"It has long been known that people with a psychotic illness such as schizophrenia are far more likely to smoke both cannabis and tobacco,” Marjorie Wallace, the chief executive of the mental health charity SANE, said.
She added that scientists believe there is a direct link between cannabis and the levels of dopamine, which is the (the chemical messenger in the brain which is related to psychotic experience).
Regular use of the substance can lead to more harm, according to the research.
Frequently smoking the drug could trigger mental health issues, hallucinations and slow down brain activity.
Researchers examined 56 patients who reported having had a psychotic episode and 43 healthy volunteers.
The average age of healthier participants was 27 while the average age of psychotic patients was 29.
Previous studies highlighted much higher incidents of psychosis among teenagers smoking pot than adults. This study, while not determining incidents of psychosis does indicate that it does occur in adults, as well as the damage to white matter.
Lead researcher Dr Paola Dazzan, from the Institute of Psychiatry, Psychology and Neuroscience at King's College London, said: “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.
“This reflects a sliding scale where the more cannabis you smoke and the higher the potency, the worse the damage will be.”
Co-author Dr Tiago Reis Marques said: “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.”
Commenting on the findings, freelance journalist Rhiannon Lucy Cosslett said legalizing the drug would give users “far greater control and choice” over the strength of what they smoked.
More than two million people in Britain smoked cannabis last year. A private analysis by the Treasury found that legalizing cannabis would raise tax revenues worth hundreds of millions of pounds and result in huge savings for the criminal justice system.
In August, government ministers debated a petition signed by 200,000 people calling for the legalization of cannabis. Its prohibition was upheld, however.