Powerful "skunk weed" cannabis causes significant damage to vital nerve fibres linking the two halves of the brain, a study has found.
The damage occurs in the corpus callosum, the structure that allows communication between the brain's left and right hemispheres.
Higher consumption of the drug caused more harm, according to the evidence. But what effect it might have on users, and whether there is any connection with psychosis - known to be associated with strong forms of cannabis - remains unclear.
Potent kinds of cannabis, popularly known as "skunk", contain high levels of the active ingredient tetrahydrocannabinol (THC). They are thought to be the most widely used forms of cannabis in the UK today.
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 fibres 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."
Two scanning techniques were used to examine "white matter" in the brains of 56 patients who had reported a first episode of psychosis, and 43 healthy volunteers.
White matter is the part of the brain made up of the "axons", or neural fibres, along which nerve signals travel.
The corpus callosum happens to be especially rich in cannabinoid "receptors", proteins that trigger biochemical effects in response to THC.
The study, published in the journal Psychological Medicine, found that frequent use of high-potency cannabis was linked to a recognised marker of white matter damage.
Co-author Dr Tiago Reis Marques, also from King's College, 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."
Dr Dazzan said there was an "urgent need" to educate health professionals, the public and policymakers about the risks associated with cannabis use.
He added: "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.''
A study involving south Londoners published in February suggested that smoking "skunk" cannabis can triple the risk of suffering a serious psychotic episode.
Psychosis is a serious mental condition characterised by hallucinations and delusions that lead to a diagnosis of schizophrenia.