Marijuana's Cardiovascular Effects Remain Unknown

Debra L. Beck

January 23, 2018

Because published data are of insufficient quality and quantity, the effects of long-term marijuana use on cardiovascular risk factors and outcomes remain unclear, authors of a new systematic analysis conclude.[1]

"Many articles in the lay press have suggested to the public that marijuana use has cardiovascular benefits, reduces blood pressure, stabilizes blood sugar levels, or improves cholesterol profiles," write the review's authors. "Our review found insufficient evidence to support these claims."

The analysis, published online January 23 in Annals of Internal Medicine, was conducted by Dr Divya Ravi (The Wright Center for Graduate Medical Education, Scranton, PA) and colleagues at the University of California, San Francisco, and the San Francisco Veterans Affairs Medical Center.

"Studying the harms of marijuana on health by means of experimental studies is unethical," Ravi told | Medscape Cardiology in an email comment. "However, researchers can design large-scale observational studies to further our understanding of its health effects."

And with more jurisdictions worldwide legalizing marijuana use, it is more important than ever to gather this information, she said.

"Given the rise in marijuana users over the decades, there is a need to perform robust longitudinal studies that adequately characterize marijuana exposure (given the different forms in which it is being used) especially among an aging population to further our understanding of the health effects of chronic marijuana use," said Ravi.

For the current review, a search of the English-language literature from January 1975 to September 2017 yielded 24 observational studies, only 2 of which were interventional and 9 of which were prospective. Any form of marijuana use, plant or pharmaceutical, was considered.  

In the 13 studies that looked at vascular risk factors (hyperglycemia, diabetes, dyslipidemia, obesity), the evidence examining the effect of marijuana use on diabetes and dyslipidemia was insufficient to permit firm conclusions.  

For example, while six studies have suggested some metabolic benefits from marijuana use, all were cross-sectional. Most had small sample sizes and included only a one-time assessment of marijuana exposure, or included patients who had only limited exposure to cannabis-related products.

In the two experimental studies on the topic—one from 1976 that included 7 participants and another from 2015 that included 13 participants—neither identified a measureable effect on metabolic parameters, including carbohydrate metabolism or lipid measures.

Eleven papers examined cardiovascular outcomes, including stroke, myocardial infarction, cardiovascular mortality, and all-cause mortality. The single study that looked at marijuana use as a potential trigger for MI was a case-crossover study published in 2001 by Mittleman et al that did show an increased risk for MI in the first hour after smoking marijuana.[2]

According to Ravi and colleagues, that study, which included 3882 patients, was "well designed," although subject to potential confounding from recall bias.

"I think the study done by Mittleman's group is still a very good study and there is a signal there that we can't ignore," said Dr Robert A Kloner (Cardiovascular Research Institute and vice president of Translation at Huntington Medical Research Institutes) in an interview.

He has published several articles on this topic over the last 15 years.

"We know marijuana can increase heart rate and blood pressure and may have ischemia-inducing effects on vasculature, which in some patients could be an issue."

Overall, however, he agrees that the current understanding of health effects of marijuana is critically lacking.

"It may have a medicinal role and help people, but we need to also know what it does to the cardiovascular system," he said.

"It costs about 1 billion dollars to bring a new drug to market in the United States, and I don't think that amount of money has been spent on research about what marijuana does to the human body," Kloner suggested.

He agrees with the investigators' call for a prospective clinical trial. "Most importantly, you need to include older patients with risk factors for cardiovascular disease, track usage over a specific length of time, and use well-defined endpoints—MI, stroke, heart disease."

"The bottom line is that marijuana is here to stay. We really just need to understand better its effects on the human body. I hope this paper will stir debate and discussion," said Kloner. 

The study was funded by the National Heart, Lung, and Blood Institute. Ravi and Kloner have disclosed no relevant financial relationships.

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