Smoking cannabis just once can change a teenager’s brain

Scientists say it could have serious implications

Teenagers who use cannabis just once or twice may end up with changes to the structure of their brain, scientists have warned.

A study, conducted by researchers at Swinburne University of Technology in Melbourne, Australia, found that there were clear differences on brain scans between teens who said they had smoked cannabis a couple of times and those who had never tried it.

Until now, research using animals to study the effects of cannabis on the brain have shown effects at low levels, leading researchers to believe that changes might occur during early stages of cannabis use.

However, the study’s lead author, Catherine Orr, says she “was surprised by the extent of the effects”.

The researchers analysed images from 46 14-year-olds who said they had used marijuana once or twice, as well as images from 46 teens who had never tried it, taking into consideration age, sex, IQ, socioeconomic status and use of alcohol or tobacco.

Upon analysing the teens' brain scans, the researchers found clear differences between the two groups, which they suspected were due to low-level cannabis use.

At this stage, the scientists couldn’t prove that marijuana led to the differences seen in the scans and acknowledged that it was possible those who chose to use marijuana had different brain structures to begin with.

To address this, the researchers analysed scans from a third group of teens that had not tried marijuana before they had their brain scans at age 14.

By the age of 16, 69 of the participants said they had used marijuana at least 10 times but their brain scans at age 14 looked no different to the brain scans of other teens who had not taken up cannabis.

This meant there could not be any inborn brain difference that predicts a person would later become a cannabis user.

What’s more, the scientists discovered there were widespread increases in the volume of grey matter – which is made up of nerve cell bodies and involved in sensory perception and muscle control - in brain regions among those who had smoked marijuana.

According to the researchers, this kind of alteration to the structure of the brain could have serious implications.

“In our sample of cannabis users, the greater volumes in the affected parts of the brain were associated with reductions in psychomotor sped and perceptual reasoning and with increased levels of anxiety two years later,” Orr said.

However, the higher volume of grey matter in cannabinoid-rich regions of the brain could be related to a normal process called “pruning” which may go awry when teens use marijuana.

As young brains develop, unnecessary or defective neurons are pruned away, Orr explained. But, when the system doesn’t work correctly, those cells remain in place.

With rates of cannabis use among adolescents increasing – it’s the most frequently used drug in Europe – concurrent with changes in the legal status of marijuana and societal attitudes regarding its use, the scientists believe the new findings are a step forward in understanding the impact it can have on developing brains, but accept that more research needs to be done.