Summary: Researchers outline the practical and scientific obstacles they encounter when studying how cannabis affects brain and heart health.
Source: American Heart Association
As more U.S. states legalize marijuana for recreational and medical use, consumption has risen—but important questions remain about how cannabis affects the brain and the cardiovascular system.
Scientists say the answers are complicated. Although cannabis has been investigated for decades, clear conclusions about its long-term effects on heart and brain health are limited by a range of methodological, legal and ethical challenges.
“Animal studies have shown potential consequences in both the brain and the heart, but human data are often ambiguous,” said Dr. Fernando Daniel Testai, a neurologist at the University of Chicago Department of Neurology and Rehabilitation. Testai was lead author on an American Heart Association scientific statement that summarized what is and isn’t known about marijuana’s effects on brain health.
Research leaders emphasize that multiple variables shape outcomes: how people use cannabis, the chemical composition of products they choose, frequency and duration of use, the user’s age, and whether the use is recreational or medicinal. These variables create complexity that is difficult to control in studies.
“People use cannabis in many different ways,” said Staci Gruber, an associate professor of psychiatry at Harvard Medical School who directs the Marijuana Investigations for Neuroscientific Discovery (MIND) program at McLean Hospital. Her team studies clinical and cognitive outcomes, brain structure and function, and quality of life in medical cannabis patients.
Marijuana—also called pot or weed—can be smoked, vaped, or consumed in edibles, oils and concentrates. Its chemical profile varies substantially: the plant contains more than 100 compounds, with the best-known being tetrahydrocannabinol (THC), which produces psychoactive effects, and cannabidiol (CBD), which does not cause a “high” and is used for some medical conditions.
“Using the word ‘marijuana’ or ‘cannabis’ as if it were a single substance is misleading,” Gruber said. “Different cannabinoids and product formulations can have distinct effects.”
Population trends reflect rising use: more than 48 million people age 12 and older reported using marijuana in 2019, about 18 percent of the U.S. population, up from roughly 11 percent two decades earlier.
Evidence suggests heavy, chronic recreational use—especially when started during adolescence—can harm cognitive function. By contrast, some adult medical cannabis patients report cognitive improvements or symptom relief that supports daily functioning. For example, a 2018 study in Frontiers in Pharmacology found certain medical cannabis users improved on measures of executive function over time without showing verbal learning or memory deficits.
The cardiovascular picture is also mixed. High levels of THC from recreational use have been associated with temporary increases in heart rate and blood pressure, but a study of older adults using medical cannabis reported lower blood pressure after three months. A 2020 American Heart Association scientific statement linked marijuana use in observational studies with higher risks of heart attack, atrial fibrillation and heart failure, but noted that definitive conclusions are limited by insufficient, inconsistent research.
Key research obstacles include:
- Product variability: Consumer products differ widely in THC and CBD concentrations and in delivery method. Research-grade cannabis supplied for studies often does not match what people buy and use privately.
- Measurement limits: Many studies rely on self-reported use, which can misrepresent actual dose, frequency and product content.
- Poly-substance use: Recreational users may also smoke tobacco or use other substances, making it hard to isolate cannabis-specific effects.
- Age and timing: Cannabis exposure during periods of brain development appears more likely to produce cognitive harm than similar exposure in adulthood.
- Regulatory constraints: Because cannabis remains a Schedule I substance at the federal level, investigators face restrictions on the products they can administer in randomized trials.
- Placebo challenges: Blinding participants in randomized trials is difficult because cannabis produces noticeable effects, complicating efforts to conduct true placebo-controlled studies.
To address product variability, Gruber’s team asks observational study participants to provide samples of the cannabis products they actually use so those samples can be analyzed. Testai said direct comparisons between medical and recreational users would improve understanding, as would standardized definitions of “chronic” or “heavy” use.
Given these hurdles, researchers say investigators must combine carefully designed clinical trials with rigorous observational studies, employ novel measurement strategies, and use creative trial designs to approximate randomized evidence where strict placebo-controlled trials are not feasible.
About this cannabis and neuroscience research news
Author: Laura Williamson
Source: American Heart Association
Contact: Laura Williamson – American Heart Association
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