True impact of weed on brain according to hundreds of thousands of scans

Plenty of studies have been carried out on the impact smoking cannabis has on the human brain which has helped build out the picture of what weed does both in the long and short term.

A recent study from the University of Colorado Anschutz which looked at the impact it has on young adults found some worrying signs of the things it can do to memory and brain activity.

Another study from a different group of researchers form the same institution turned up some surprising results when they looked at what was happening to older generations.

Meanwhile, Dr Daniel Amen who has conducted hundreds of thousands of scans on brains told the New York Post that cannabis users 'demonstrate widespread reductions in cerebral blood flow, particularly in regions critical for memory, coordination, emotion and executive function'.

He pointed to the University of Colorado study on young adults to show the effects it had on the human brain.

Perhaps unsurprisingly, studies of what cannabis does to the brain indicated it impaired several functions of your noggin (Getty Stock Photo)

Impact on brain functions

That study looked at adults aged between 22 and 36 (congratulations to those in their mid 30s who realised they're still young) who had varying levels of cannabis consumption.

Heavy use was defined as over 1,000 times in the course of their life, non use was below 10 times and anything between them was moderate.

Researchers gave this group of over 1,000 young adults seven tasks to complete and scanned their brains, finding a particular impact on memory.

They found 63 percent of heavy users showed signs of reduced brain activity during a working memory task, and 68 percent of recent users showed similar results.

Reduced brain activity appeared to impair a person's decision-making, memory, attention and emotional processing, so a lot of what your brain is responsible for is going to be affected.

A person's decision making, memory, attention span and mental health were all affected (Getty Stock Photo)

Trying to quit at short notice could pose similar problems to brain activity too.

Assistant professor Joshua Gowin said: "People need to be aware of their relationship with cannabis since abstaining cold turkey could disrupt their cognition as well. For example, heavy users may need to be more cautious.

"There are a lot of questions we still need answers to regarding how cannabis impacts the brain.

"Large, long-term studies are needed next to understand whether cannabis use directly changes brain function, how long these effects last and the impact on different age groups."

Dr Amen also warned that long term use could have a negative impact on one's mental health as well, saying imaging of the brain also showed decreased activity in the amygdala.

He warned this could lead to 'emotional blunting in some individuals', which could result in 'increased rates of anxiety, paranoia and psychosis'.

It can mess with your ability to remember things, but doesn't harm your sense of familiarity as much so you can get 'false memories' (Getty Stock Photo)

False memories

Other studies such as the one conducted by researchers at Washington State University have warned that alongside the memory issues caused by cannabis, and more specifically its main psychoactive compound THC, it can also result in 'false memories'.

Researchers there found THC weakens a person's recollection but doesn't impair their sense of familiarity, so people can remember things incorrectly but be sure that's what actually happened.

"Relative to placebo, cannabis increased susceptibility to false memories and detrimentally impacted verbal memory (immediate, delayed, working), visuospatial memory (immediate, delayed), event-cued prospective memory, source memory, and temporal order memory," the researchers wrote.

"There were no significant differences between the moderate and high dose groups."

Associate professor Carrie Cuttler, who is a co-author of the 2026 study, told National Geographic: "You are activating certain pathways to create a memory and to recall something, you are trying to reactivate that same pathway.

"If you blast the system with THC, the THC hijacks the system."

She explained that staying away from cannabis for a month could lead to a 'complete rebound' and sort this out, and noted that the impact of alcohol on memory was worse.

Some studies have indicated cannabis use might have been beneficial for older brains, but scientists warned a lot of that drug use was from different kinds of cannabis (Getty Stock Photo)

Potential positives for older users

Another University of Colorado study looking at the impact on older people analysed the results from over 26,000 cannabis users in the UK.

"More older adults are using cannabis. It's more widely available and is being used for different reasons than in younger folks—such as for sleep and chronic pain," said lead author of the study, Anika Guha Ph.D., during a Q&A with Medical Press.

What they found was that among older adults more cannabis use in the lifetime was 'generally associated with larger brain volumes and better cognitive function'.

However, Dr Guha warned that it was 'not a case of cannabis being all good or all bad'.

She explained it mattered a lot how people were using cannabis and what kind of weed they were smoking, especially as far as THC amounts were concerned.

The doctor also pointed out that many people involved with the study had historical cannabis usage but weren't still using the drug, and that cannabis back then 'looked very different from what's available today'.

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