THE WASHINGTON POST: Seven unexpected takeaways from the newest research on cannabis and brain effects

Stephen Lankenau has spent years studying how people use cannabis in everyday life. As director of Drexel University’s Medical Cannabis Research Centre, he has watched legalisation spread, products grow stronger and daily use become increasingly common.

Yet one of the most basic questions remains surprisingly difficult to answer: How much cannabis is too much for the brain?

Social rituals around alcohol help define moderation - a beer after work, a glass of wine with dinner - and numerous studies have looked at safe alcohol limits. One federally commissioned paper published this month in the Journal of Studies on Alcohol and Drugs found that anything more than one drink a day increases mortality. But cannabis is entering mainstream life with few shared rules or routines. Researchers are trying to understand what problematic use looks like and whether new habits and cultural norms could help prevent it.

The question is becoming more urgent as cannabis use rises, particularly among younger adults, who tend to use it most heavily and are the most likely to experiment with increasingly potent products.

Doctors report increasing numbers of patients arriving in emergency rooms with confusion, paranoia, rapid heart beats, dizziness and other signs of cannabis intoxication. One study found that cases of cannabinoid hyperemesis syndrome, a condition involving severe, repeated vomiting, quadrupled among adults ages 18 to 35 between 2016 and 2022.

Recreational cannabis is now legal in 24 US states and DC while in Australia it is legal for personal use in the ACT, South Australia and the Northern Territory. In 2002, about 25 million Americans reported using cannabis in the past year, according to the National Survey on Drug Use and Health. By 2023, that number had climbed to nearly 70 million. Daily or near-daily use has climbed even faster than occasional use, and adults over 35 are now among the fastest-growing groups of users.

Sorting out what constitutes “too much” is tricky. Most research has not been designed around recreational use, and today’s consumers can choose from products ranging from high-potency vapes and concentrates to gummies, drinks and tinctures. But the science is advancing quickly. Some of the findings challenge long-held assumptions about the benefits of cannabis and health. And contemporary cannabis research is increasingly converging on three big variables: How often? How young? How strong is the drug?

1. Shifting potency

When marijuana entered mainstream American life in the 1960s, it quickly became attached to a familiar set of images: hippies, anti-war protests, youthful rebellion, experimentation. The debate over its effects was often blunt. Cannabis was either dangerous or liberating, mind-clouding or mind-expanding.

Interpreting cannabis research today is challenging because marijuana is far more potent than the products used by previous generations. In the 1970s and ’80s, most cannabis flower contained relatively modest levels of THC - often in the low single digits. Today, many commercial products routinely exceed 20 per cent THC. Concentrated products such as vape cartridges, waxes and “dabs” can reach 70 to 90 percent.

THC, or tetrahydrocannabinol, is the compound primarily responsible for cannabis’s psychoactive effects, including intoxication, paranoia and, in some users, psychotic symptoms.

Some scientists think highly concentrated THC may produce larger dopamine surges and more profound disruptions in perception and salience processing - the brain’s ability to determine what deserves attention.

A growing number of studies have linked frequent use of high-potency cannabis to altered brain connectivity, working-memory deficits and increased risk of psychosis-like symptoms, particularly among younger users and people with underlying vulnerabilities.

2. Overstated mental health benefits

Many adults, especially older ones, use cannabis for medical reasons such as to address pain, sleep or anxiety. But two recent studies - one in JAMA Internal Medicine and another in the Lancet - cast doubts.

The JAMA Internal Medicine report looked at previous studies related to PTSD, anxiety, depression, ADHD, bipolar disorder and other conditions, and found that the current evidence does not support the use of cannabis for any of them. On the other hand, the researchers warned, use of cannabis “demonstrates substantial risks of adverse effects.”

The Lancet paper involved a systematic review and meta-analysis of 54 randomised controlled trials with nearly 2500 participants involving a wide range of mental health and other conditions. It found some evidence of a reduction in insomnia, tic or Tourette’s syndrome, and autism spectrum disorder, but said “the quality of this evidence was generally low.”

“Given the scarcity of evidence, the routine use of cannabinoids for the treatment of mental disorders … is currently rarely justified,” the authors concluded.

3. Cognitive effects may vary by age

One of the more surprising shifts in cannabis research is that moderate use in adulthood may not impair cognition nearly as much as scientists once feared.

A widely discussed 2024 study published in JAMA Network Open, which examined middle-aged and older adults, found no major association between moderate cannabis use and cognitive decline across several domains after a year of use. But the researchers noted participants generally used lower-potency products and consumed them less than daily.

Carl Hart, a Columbia University psychologist who has studied cannabis and other drugs for decades, argues that the adult brain may be far more resilient to marijuana than many people assume. In one experiment, Professor Hart and his colleagues offered adult participants the chance to earn money by performing well on a maths test. Participants could choose whether to smoke cannabis beforehand.

“Invariably they did not want to smoke because they wanted to make as much money as possible,” Professor Hart said.

In this study and others, Professor Hart found that smoking cannabis had a minimal impact on accuracy of complex cognitive tasks. To Professor Hart, the findings challenge one of the oldest assumptions about marijuana - that it inevitably drains motivation or ambition.

“Think of all the many people who take cannabis,” he said, “there are some people not motivated to do jack, but that isn’t because of cannabis. And there are a wide range of people who have done incredible things in the world while on cannabis. But the notion just won’t die.”

Other clues about what should be considered safe use come from a large 2025 study published in JAMA Network Open.

Researchers analysing brain scans from more than 1,000 young adults ages 22 to 36 found that the clearest cognitive effects appeared among heavy lifetime users - people who had used cannabis more than 1,000 times. They showed reduced brain activity during tasks of working memory - which involves holding and using information in real time - while moderate users showed far fewer differences. The findings suggest that frequency and intensity of use may matter more than occasional consumption alone.

Notably, working memory was the only cognitive domain among seven tested that showed a statistically significant association with heavy cannabis use, suggesting that any effects on the brain may be more targeted than wide-reaching.

4. Teenage brains appear most vulnerable

Studies going back decades have found that adolescents who use cannabis regularly tend to earn lower grades and graduate from high school at lower rates, a pattern sometimes referred to as “amotivational syndrome.”

In February, a JAMA Health Forum study that followed roughly 460,000 adolescents ages 13 to 17 into young adulthood found that using cannabis in the previous year was associate with double the risk of later psychotic disorders and bipolar disorders when they reach age 26. In 2025, a JAMA Psychiatry study of young adults with cannabis use disorder found alterations in the dopamine-related brain system that were similar to what is seen in psychosis. These observation studies cannot prove causation, but the links have worried scientists.

Cannabis use disorder involves an inability to quit and psychological dependence and physiological withdrawal symptoms. The Centres for Disease Control and Prevention says three out of 10 people who use cannabis have it.

“All of these very serious neuropsychiatric symptoms are associated with motivation,” said Bertha Madras, a professor of psychobiology at Harvard Medical School.

During puberty and early adulthood, neural connections are rapidly being formed, strengthened and pruned. In fact, brain-imaging research has linked cannabis use before age 16 to changes in white matter, the neural pathways that help different regions of the brain communicate.

One paper from 2021 found small declines in IQ in youths who used cannabis frequently.

Madras has conducted animal experiments involving adolescent rats and primates given daily marijuana exposure comparable to human use. She found pronounced inflammation in a part of the brain involved in emotional regulation and stress control.

“We didn’t see it in the adult brain. It was only the adolescent brain,” she said. “We began to speculate this observation may help explain why the drug has certain adverse effects on adolescents more than in adults.”

Some studies suggest that cannabis use beginning at a younger age is associated with more persistent changes in executive function and IQ-related measures, but the findings remain subject to debate because it is difficult to fully account for socioeconomic, environmental, genetic and other confounding factors.

5. Chance of a cognitive rebound

The idea of taking a “tolerance break” - a temporary pause in cannabis use - has long been popular among regular users. Over time, the body can become less responsive to THC, the psychoactive compound in cannabis, meaning people may need larger amounts to achieve the same effects. Many long-term users notice that cannabis feels less potent than it once did, even as their consumption increases.

The field remains complicated because many studies are small and it can be difficult to separate the effects of cannabis from those of alcohol, nicotine and other substances.

A 2021 review published in Current Behavioural Neuroscience Reports found that some cognitive deficits associated with cannabis use - particularly those involving attention and working memory - may improve after sustained abstinence, especially among adults and lighter users. Similarly, a 2018 study in the Journal of the International Neuropsychological Society found that adolescents and young adults who abstained from cannabis for two weeks showed measurable improvements in attention.

Brain imaging studies have also provided encouraging signs. Several have found that altered patterns of brain connectivity in heavy cannabis users may partially normalise after a period of abstinence, particularly in networks involved in executive function, reward processing and emotional regulation.

These findings do not necessarily mean that all cannabis-related brain changes are reversible, nor do they establish how long recovery may take. But they suggest that for some people, reducing or stopping cannabis use may allow some cognitive function to rebound over time.

6. Signs of protecting older brains

Scientists are increasingly exploring whether certain cannabinoids might help protect ageing brains by reducing inflammation and cellular stress - processes believed to play a role in diseases such as Alzheimer’s and Parkinson’s. The research is still early, and most of the evidence comes from laboratory studies rather than human trials.

A 2024 study from the Salk Institute drew attention for its focus on a little-known cannabinoid called cannabinol, or CBN, which forms as THC ages and breaks down over time.

The researchers studied a form of cell death that has been increasingly linked to neurodegenerative disease. In conditions such as Alzheimer’s and Parkinson’s, neurons often begin to die after their mitochondria - the tiny structures that produce energy inside cells - stop functioning properly.

Using mouse neurons, human brain-cell cultures and fruit-fly models, the Salk team found that CBN appeared to help protect neurons by preserving mitochondrial function and reducing oxidative stress, a damaging process associated with ageing and neurodegeneration.

The findings generated excitement among some researchers because they suggest that stabilising those cellular energy systems might someday help slow aspects of neurodegenerative decline. But scientists caution that the work remains far from proving that cannabis-derived products can prevent dementia or Parkinson’s disease in humans.

7. Setting boundaries matters

Researchers studying cannabis have noted that while some users develop problematic patterns of use, most do not. This led scientists to ask what successful users do differently. The idea stems from the long-standing “drug, set and setting” theory, which argues that a person’s experience with a substance is shaped not only by the drug itself but also by their mindset (“set”) and the environment in which they use it (“setting”).

In a paper published in December, Professor Lankenau and colleagues looked a nine years of data on young cannabis users in Los Angeles.

They found that most participants fell into an “uncontrolled” group that used cannabis without consistently following self-imposed rules, while a smaller “controlled” group practised rules such as not using before work or school and not driving while high. Those in the controlled group used cannabis less frequently and were less likely to show signs of problematic use.

A key takeaway, he said, is that moderation may depend less on the amount of cannabis someone uses than on whether they develop consistent boundaries around when, where and why they use it. The authors argue that understanding and promoting these “controlled use” practices could become an important public health strategy in the era of legal cannabis.

Professor Lankenau argued that as cannabis becomes more widely available, public health agencies - not cannabis companies - need to take the lead in educating consumers about potency, dosing and potential risks. Because in the US many people can now purchase high-potency products directly from dispensaries without consulting a physician.

“The more educated consumers are, the less negative effects we will see,” Professor Lankenau said.