Just one or two joints is enough to change the structure of a teenager’s brain, scientists have warned.
And the drug could cause changes affecting how likely they are to suffer from anxiety or panic, according to a study.
Researchers found 14-year-old girls and boys exposed to THC – the psychoactive chemical in cannabis – had a greater volume of grey matter in their brains.
This means the tissue in certain areas is thicker, and it was found to be in the same areas as the receptors which marijuana affects.
Experts said thickening of brain tissue is the opposite of what usually happens during puberty, when teenagers’ brain matter gets thinner and more refined.
Researchers did scans of teenagers’ brains and discovered those who had been exposed to small amounts of marijuana (top row) had thicker regions of the brain (indicated by more orange and yellow tissue) than those who had never smoked cannabis (bottom row)
Researchers from the University of Vermont scanned the brains of teenagers from England, Ireland, France and Germany to study marijuana’s effects.
They found differences in the volume of grey matter in the amygdala and the hippocampus.
These sections are involved with emotions, fear, memory development and spatial skills – changes to them suggests smoking cannabis could affect these faculties.
Scientists said theirs is the first evidence to suggest structural brain changes and cognitive effects of just one or two uses of cannabis in young teenagers.
And it suggests as teenagers brains are still developing, they may be particularly vulnerable to the effects of THC.
THC, full name tetrahydrocannabinol, is the chemical in marijuana which makes people high and is what makes it illegal in the UK.
‘Consuming just one or two joints seems to change grey matter volumes in young adolescents,’ said study author Professor Dr Hugh Garavan.
‘The implication is that this is potentially a consequence of cannabis use. You’re changing your brain with just one or two joints.
‘Most people would likely assume that one or two joints would have no impact on the brain.’
What changes the increased brain volume directly causes is unclear, but the researchers said it is important to understand cannabis’s effects in detail.
This is especially so in the US, where more states are legalising the drug and a view of it being harmless is spreading, they said.
Professor Garavan said cannabis use appears to produce the opposite effect on brain matter of what usually happens during puberty.
IS ALCOHOL OR CANNABIS WORSE FOR THE BRAIN?
Alcohol damages the brain more than cannabis, research suggested in February 2017.
Unlike booze, marijuana does not affect the size or integrity of white or grey matter in the brain, even after years of exposure, a study found.
Grey matter enables the brain to function, while white controls communication between nerve clusters.
Study author Professor Kent Hutchison, from the University of Colorado Boulder, said: ‘While marijuana may also have some negative consequences, it definitely is nowhere near the negative consequences of alcohol.’
The scientists add, however, research into cannabis’ mental effects are still very limited.
Lead author Rachel Thayer said: ‘Particularly with marijuana use, there is still so much that we don’t know about how it impacts the brain.’
In the US, 44 percent of those aged 12 or over have used cannabis at some point in their lives.
Although their findings appear positive, the researchers also add there is a long way to go before cannabis will likely be broadly legalised.
Many are still concerned as to how the class-C drug affects people of different ages, manages pain and causes addiction.
He said a typical adolescent brain undergoes a ‘pruning’ process in which it gets thinner, rather than thicker, as it refines its connections.
‘One possibility is they’ve actually disrupted that pruning process,’ he said.
Previous studies have focused on heavy marijuana users later in life and compared them against non-users.
Few have looked at the effects of the first few uses of a drug.
Another of the study’s authors, Catherine Orr, now a lecturer at Swinburne University of Technology in Australia said: ‘Rates of cannabis use among adolescents are high and are increasingly concurrent with changes in the legal status of marijuana and societal attitudes regarding its use.
‘Recreational cannabis use is understudied, especially in the adolescent period when neural maturation may make users particularly vulnerable to the effects of THC on brain structure.’
The study, part of a long-term European project known as IMAGEN, involved 46 teenagers who used recreational marijuana once or twice by the age of 14.
They reported how many joints they had smoked and had brain scans.
It also involved 69 teenagers who used the drug at least 10 times between the ages of 14 and 16, and 69 who had not touched the drug by age 16.
Scientists also assessed them for signs of various mental disorders including ADHD, anxiety, depression and panic disorder.
Dr Orr said: ‘Of the behavioural variables tested, only sensation seeking and agoraphobia differed between the cannabis users and controls. And these factors were not related to greater grey matter differences.’
The researchers said the area of the brain which cannabis interacts with is particularly important for brain development in adolescence, suggesting teenagers could be particularly affected by THC.
Dr Orr concluded: ‘Almost 35 per cent of American 10th graders have reported using cannabis and existing research suggests that initiation of cannabis use in adolescence is associated with long-term neurocognitive effects.
‘We understand very little about the earliest effects of cannabis use, however, as most research is conducted in adults with a heavy pattern of lifetime use.
‘This study presents evidence suggesting structural brain and cognitive effects of just one or two instances of cannabis use in adolescence.’
The study was published in The Journal of Neuroscience.