Cannabis slows teen brains but a ‘three-day detox’ combats this

Cannabis does make teenagers’ and young adults’ brains perform more poorly, but the effects fade if they stop using it for just three days, a new study reveals.

As cannabis becomes more widely legal in the US, it becomes more accessible to teenagers, and a quarter of people under 20 already use marijuana.

With low funding and red tape blocking cannabis research, experts have warned that its adoption has been outpacing the ability of science to analyze its health consequences. 

University of Pennsylvania researchers reviewed 69 previous studies and found that teenagers that use cannabis do not perform as well on tests of their cognitive functions, but can easily recover these abilities within 72 hours of quitting.

Cannabis does impair teenagers’ brains, but only for a short while. After 72 hours without a toke, their mental performances return to normal, new research suggests 

Marijuana is legal for medical use in 29 states, and for recreational use in nine states. Now, even states that have been traditionally against legalization – like Utah and Oklahoma – are pushing to make it available.

All of these, however, restrict its use to those over 21, in part due to concerns over marijuana’s possible effects on developing brains.

Previous research – though it has been sparse – has shown signs that the more heavily adults used cannabis, and the earlier they started, the more noticeable the changes to their brains were.

But just in time for 4/20, the new study suggests that young marijuana-lovers can exhale with relief as they are not doing lasting damage to their brains.  

Marijuana has two compounds that act on the brain: tetrahydracannabinol (THC) and cannabidiol.

The brain has a network called the endocannabinoid system, which helps to regulate our mood, appetite, memory had experience of pain.

The system’s primary goal is to help us maintain homeostasis, or a sort of full-body physiological balance.

Sometimes to strike that balance, the communication between cells need to be slowed down, and the brain has two chemicals that that act this way on the endocannabinoid system.

THC binds with receptors in the endocannabinoid system, and has a similar slowing effect on cellular communications, but, in large quantities it can overwhelm the brain and throw off the balance the system is meant to maintain.

But when scientists have tried to study whether marijuana’s effects on the brain can be long-lasting or permanent, the results have been mixed, and gathered from small studies.


Tetrahydrocannabinol (THC) and cannabidiol (CBD) are both derived from the cannabis plant. 

Together, they are part of the cannabinoid group of compounds found in hashish, hash oil, and most strains of marijuana. 

THC is the psychoactive compound responsible for the euphoric, ‘high’ feeling often associated with marijuana.

THC interacts with CB1 receptors in the central nervous system and brain and creates the sensations of euphoria and anxiety. 

CBD does not fit these receptors well, and actually decreases the effects of THC, and is not psychoactive. 

CBD is thought to help reduce anxiety and inflammation. 

The University of Pennsylvania study authors decided to look for patterns among developing teenagers’ and young adults’ brains by conducting a meta-analysis of nearly 70 studies dating back to the 1970s and done as recently as last year.

They compared how well 2,152 young cannabis users performed on cognitive tests to the scores of 6,575 people who did not or had only very rarely used marijuana.

The differences among them were slight, but consistent: those who smoked or ingested weed most frequently scored a bit lower on these tests.

But the scientists found that if cannabis-users stopped for just 72 hours, their cognitive functioning started to improve again and return to normal, suggesting that marijuana’s brain effects are reversible for them.

‘We found little evidence for more severe effects with cannabis use at earlier ages or specifically in adolescence,’ the authors wrote.   

Their study could not draw clear conclusions about how this recovery process may change with age. 

The researchers referred to previous work that suggests that by ate 38, there are cognitive deficits apparent in people who started using marijuana young and kept using it frequently throughout their lives, though the study was a small one. 

‘In light of the changing perceptions of cannabis use and an evolving policy landscape surrounding cannabis, understanding the potential risks of cannabis use for mental health and brain functioning is of paramount importance,’ the researchers wrote. 

They noted that in order to get a clearer picture of how the brain is affected by marijuana over time, longer, larger studies are necessary. 

Their own research was limited by the relatively little background data they had on their research subjects. 

The authors suggest that we need to establish better baselines for study participants’ cognitive abilities and called for future research to look at the interplay of genetics and the impact of marijuana on individuals.  

‘There is likely substantial variability in risk for cognitive and mental health problems associated with cannabis use, and research into such factors (e.g., genomic profiles) will be crucial to avoid unnecessary cannabinoid-related adverse effects,’ the researchers wrote.