Cannabis really does damage the brain even if it is medicinal, British scientists warned.
Experiments found it causes long term memory loss – which has implications for dope smokers and those who use the drug to combat epilepsy, multiple sclerosis and chronic pain.
Scans showed marijuana impairs function in key regions of the brain involved in learning.
Mice exposed to the cannabinoids THC and CBD, chemicals found in the plant, could not even discriminate between a familiar and novel object.
The former is the active ingredient that gets people ‘high’, while the latter is responsible for the therapeutic effects from relieving pain to soothing anxiety.
Study leader Dr Neil Dawson, of the University of Lancaster, said pot smokers must be made aware of the risks.
He said: “This work offers valuable new insight into the way in which long-term cannabinoid exposure negatively impacts on the brain.
“Understanding these mechanisms is central to understanding how long-term cannabinoid exposure increases the risk of developing mental health issues and memory problems.”
He also highlighted the relevance to those using cannabinoid-based drugs to treat medical conditions.
Dr Dawson said: “Cannabis-based therapies can be very effective at treating the symptoms of chronic diseases such as epilepsy and multiple sclerosis, and dramatically increase the quality of life for people living with these conditions.
“We need to understand the side effects these people may experience so we can develop new interventions to minimise these side effects”.
Working with colleagues in Portugal, his team found mice exposed to cannabis over a long period had “significant memory impairments”.
There is little understanding of the potential side effects of long-term cannabinoid exposure.
But it is already known heavy, regular cannabis use raises the risk of developing mental health problems including psychosis and schizophrenia.
More and more people are regular users due to its legalisation for medical reasons in several countries. More potent varieties are also available for recreational users.
When the mice were given a cannabinoid drug it reduced the ability of brain regions involved in learning and memory to communicate with each other.
Dr Dawson and colleagues said this suggests the underlying cause for cannabis’ effect on memory.
Co author Professor Ana Sebastiao, of the University of Lisbon, said: “Importantly, our work clearly shows that prolonged cannabinoid intake, when not used for medical reasons, does have a negative impact in brain function and memory.
“It is important to understand that the same medicine may re-establish an equilibrium under certain diseased conditions, such as in epilepsy or MS, but could cause marked imbalances in healthy individuals.
“As for all medicines, cannabinoid based therapies have not only beneficial disease-related actions, but also negative side effects.
“It is for the medical doctor to weight the advantages of the therapy, taking into consideration quality of life and diseases progression, against the potential side effects.”
Last week Government advisers in the UK declared doctors should be able to prescribe medicinal cannabis.
The Advisory Council on the Misuse of Drugs (ACMD) agreed cannabis does possess a medicinal benefit, in a review sent to to Home Secretary Sajid Javid.
He commissioned the review after two high profile cases, including that of epileptic 12-year-old Billy Caldwell, whose mother had seven bottles of cannabis oil that helped combat his seizures confiscated at Heathrow Airport.
For years scientists have warned smoking cannabis can lead to mental health problems, such as schizophrenia.
Studies have shown cannabis can also shrink memory-related structures in the brain, most notably the hippocampus.
The new findings published in the Journal of Neurochemistry shed fresh light on the dangers.