Compound statement

Scientific statement highlights uncertainty about health effects of marijuana

Despite the perception that marijuana is harmless, some scientific evidence challenges that belief, and many questions remain unanswered about its impact on brain health, according to a new scientific statement from the American Heart Association published today in the Association’s Stroke magazine. This scientific statement will be presented and discussed at a symposium at the Association’s International Stroke Conference in New Orleans today at 7 a.m. CT/8 a.m. ET. A scientific statement from the American Heart Association is an expert review of current research and may inform future clinical practice guidelines.

There is a lot of uncertainty in the medical community about the health effects of marijuana. This scientific statement is intended to guide healthcare professionals in a balanced and intentional discussion with patients about the known and unknown potential effects of marijuana on brain health.”

Fernando D. Testai, MD, Ph.D., FAHA, Editorial Group Chair, Professor of Neurology and Rehabilitation, University of Illinois at Chicago

This is the Association’s first scientific statement on cannabis and brain health, following a statement on marijuana and heart health, released in August 2020. Both statements are important because the use of marijuana in the States States is increasing, particularly among adolescents and young adults, with about one-third of 12and students and nearly half of college students reported using marijuana in 2018. Additionally, medical and/or recreational marijuana use has been legalized or decriminalized in many states in the United States over the past two decades , and the concentration of tetrahydrocannabinol (THC, the psychoactive component of marijuana) in cannabis products has increased dramatically, from around 4% in 1995 to 15% in 2018.

The most studied chemicals in cannabis are THC and CBD. THC is the compound in marijuana that makes you feel high. CBD (cannabidiol) has antioxidant and anti-inflammatory properties but does not have psychoactive effects. The potential therapeutic benefits of CBD continue to be studied in clinical trials.

The U.S. Drug Enforcement Agency (DEA) and Food and Drug Administration (FDA) classify cannabis as a Schedule I controlled substance, on par with heroin and LSD, for having “high drug potential.” ‘abuse and little or no medical benefit’. In contrast, CBD is legal when derived from hemp, which is the same species of plant as cannabis and contains less than 0.3% THC.

To fully understand the potential impact of marijuana, it is important to know that the human body naturally produces compounds called endocannabinoids that are similar to those in marijuana. Endocannabinoids are involved in the regulation of many bodily processes throughout life (including learning, memory, pain control, and sleep), and the action of endocannabinoids is essential for prenatal brain development. and brain maturation during adolescence.

Endocannabinoids, along with THC, can attach to neurons in the brain through molecules called cannabinoid receptors. When THC activates cannabinoid receptors in the brain, it can disrupt the normal actions of endocannabinoids. “These receptors are highly concentrated in areas of the brain related to cognition,” Testai said.

According to the statement, previous animal studies (in rodents) indicate that prolonged exposure to THC impairs memory and learning, and impacts brain development and maturation specifically in cases of exposure at certain life stages:

  • During prenatal life, an important period for brain development, THC disrupts normal signaling pathways in the endocannabinoid system and can alter thinking, emotional behavior and stress response in offspring.
  • During adolescence, an important period for brain maturation, THC alters the structure and function of brain circuits, particularly in areas involved in cognition, emotional regulation and social behavior (such as the prefrontal cortex and hippocampus ).

“The data obtained in these animal studies demonstrate that disruption of endocannabinoid pathways leads to behavioral and cognitive abnormalities, such as poorer memory and learning ability and increased sensitivity to stress. In addition, there may be vital periods of life – gestation and adolescence – when the brain can be especially vulnerable to the impact of THC,” Testai said.

While the exact timing and amount of marijuana exposure is more easily controlled in animal studies, as well as control of animals’ social and environmental conditions, human research studies cannot reproduce similar strict parameters. Thus, the results of existing studies in humans have been mixed, but raise similar concerns about the impact of marijuana exposure on brain health. Among the human studies summarized in the scientific statement, the results included:

  • While actively using marijuana, people performed worse on highway driving tests when using THC-dominant marijuana, compared to when using CBD-dominant marijuana or no marijuana .
  • In young adults who were followed for 25 years as part of a heart disease research project, scores on verbal memory tests decreased in correlation with more years of self-reported exposure to marijuana.
  • There were more psychological problems and lower cognitive function in children (average age 9) whose mothers reported using marijuana during pregnancy.
  • Marijuana use during adolescence has been associated with thinning in an area of ​​the brain involved in cognition (the prefrontal cortex), with greater exposure to marijuana being associated with greater thinning. However, other studies have detected no difference.
  • Structural changes in the brain were visible in some studies comparing marijuana users and non-users. Specifically, there was a thinning of brain areas important for orchestrating thoughts and actions, or a decrease in volume in an area of ​​the brain important for memory. Other studies comparing cognitive testing and brain imaging have found no difference between marijuana users and nonusers.
  • Cannabis users have been found to have an increased risk of stroke caused by a clot, with one study finding 17% more and another 24% more stroke in cannabis users.

The statement also highlights many open questions about the impact of cannabis on brain health, including:

  • Does the impact of marijuana on brain health differ by age?
  • How does marijuana interact with other substances such as prescription drugs? This is of particular concern in older people who may use multiple medications such as blood thinners, antiarrhythmics, or anticonvulsants to treat other chronic health conditions.
  • Do the effects of marijuana differ depending on whether it is used recreationally or prescribed for the treatment of a specific medical condition?
  • How much marijuana is too much? In older research studies conducted when marijuana was illegal in all US states, there may have been significant under-reporting of the frequency of marijuana use.
  • Do different types of marijuana (such as higher THC levels or synthetic cannabinoids) impact the brain differently?
  • Are there differences in brain health depending on whether marijuana is smoked or consumed in an edible?

“Our understanding of the effects of marijuana on the brain is imperfect, and human research in this area is ongoing. Yet the results of recent animal studies challenge the widely accepted idea that cannabinoids are harmless and urge caution when using marijuana, especially during pregnancy or during adolescence,” Testai said.

This scientific statement was prepared by the volunteer writing group on behalf of the Stroke Council’s American Heart Association Stroke Brain Health Sciences Subcommittee; the Arteriosclerosis, Thrombosis and Vascular Biology Council; Cardiovascular and Stroke Nursing Council; the Council on Lifestyle and Cardiometabolic Health; and the Peripheral Vascular Disease Council. The American Academy of Neurology has confirmed this scientific statement as an educational tool for neurologists.


American Heart Association

Journal reference:

Testai, FD, et al. (2022) Marijuana Use: Effect on Brain Health: A Scientific Statement from the American Heart Association. Stroke.