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expert reaction to study looking at marijuana use and various measures of heart disease

Publishing in the European Journal of Preventative Cardiology, researchers looked at the effect of marijuana on cardiovascular
and cerebrovascular mortality and reported that longer duration of marijuana use is associated with an increased risk of death from hypotension.


Dr Graham Wheeler, Bayesian Medical Statistician, UCL, said:

“People who reported using marijuana once were classed as current marijuana users, so someone who tried marijuana once 15 years ago appeared in the data as someone who has used marijuana for the last 15 years.  Data were not collected on the amount or frequency of marijuana usage, so we don’t actually know how yearly usage is associated with the risk of death from hypertension.

“Age of participants was classified as either under or over 25 years.  This means we lose information about how the risk of death from hypertension may change with age, and whether this affects the reported association attributed to using marijuana.

“No data on physical activity were collected for this research, despite regular physical activity being shown to help reduce blood pressure and recommended by British, European, and American guidelines on hypertension management.

“Having these data would help us assess whether there is still evidence of the reported association between using marijuana and death from hypertension.

“No association was reported between marijuana usage and deaths from heart disease or cerebrovascular diseases.  Factors known to affect blood pressure, such as alcohol consumption and smoking, were not associated with high blood pressure mortality in this study.”


Prof Terrie Moffitt, Chair in Social Behaviour and Development & Professor of Psychiatry, Institute of Psychiatry Psychology & Neuroscience, King’s College London (IoPPN), said:

“As the authors acknowledge, this study’s weak link is its measure of cannabis exposure.  The study defined use as ‘yes’ to a question about having ever tried cannabis, and then assumed that respondents who said yes had continued using for years thereafter in a way that could harm health.  However, prospective cohort studies show that fewer than 10% of people who ever try cannabis go on to use it regularly.  Of all the many studies of cannabis effects, no prior study has ever reported a harmful result from short term recreational use (as opposed to persistent frequent use).  This situation leaves unknown what the exposure variable in this study really is.”


Dr Amitava Banerjee, Senior Clinical Lecturer in Clinical Data Science and Honorary Consultant Cardiologist, UCL, said:

“This study adds to evidence for long-term cardiovascular risks of marijuana use.  A systematic review last month1 showed that 29 previous observational studies have looked at this question, and suggested that the link between marijuana and stroke was stronger than any other cardiovascular diseases.  Recent research from Australia2 also found that marijuana is associated with stiffness of the arteries and higher ‘vascular age’, particularly at high doses.  It is important to note that neither the current study nor other retrospective observational studies can look at causation.

“In a US population, the authors report that marijuana was associated with a three-fold increased risk of hypertension-related deaths, but not those from heart disease or strokes.  However, this is a relatively small study, in that the number of overall individuals is small (1213 individuals) and the number of deaths, particularly from hypertension (40 of 332 deaths) are low.  Therefore, an association with deaths from heart disease and stroke cannot be excluded.  In addition, the information about exposure to marijuana was based on a single question in a questionnaire at one time point, so it is difficult to draw conclusions about the threshold of marijuana use above which there is an associated risk.

“Further studies are necessary using more detailed, validated data about marijuana use over time in a larger population.  In addition, Mendelian randomisation, a research method which uses the random allocation of genes to create ‘natural trials’ could be applied to the question of whether marijuana is involved in causation of cardiovascular disease, hypertension or otherwise.  Meanwhile, both the public and health professionals should be aware that there is a growing body of evidence of increased risk of high blood pressure and cardiovascular disease with marijuana use, independent of the risk associated with tobacco.”



Ian Hamilton, Lecturer in Mental Health, University of York, said:

“Despite the widely held view that cannabis is benign this research adds to previous work suggesting otherwise.  In particular the risk of raised blood pressure which can lead to premature death.

“It is difficult to translate these findings into the UK as the majority of these American participants in this research did not combine tobacco with their cannabis; in the UK most users include tobacco in their cannabis joint.  Smoking a joint with tobacco raises the risk of developing problems with your circulation and heart.  In that sense it is the tobacco that is a greater threat to health than cannabis.  The health priority and message should be that we encourage people to leave tobacco out of their joints and consider alternative ways of consuming cannabis such as vaping or edibles.

“Unfortunately this research doesn’t help us know who specifically is at risk as it doesn’t distinguish between those who used cannabis once and those who had used frequently.  This matters as the impact of cannabis on other areas of health is known to increase with the amount and frequency with which you have used it.”


Dr Tim Chico, Reader in Cardiovascular Medicine / consultant cardiologist, University of Sheffield, said:

“Given an increasing trend to decriminalise use of marijuana, it is important to understand the health risks of its use.  This study attempted to work out the risk of various heart diseases from smoking marijuana, but its findings are far from definite.  The researchers assumed that if people answered ‘yes’ to the question ‘have you ever used marijuana or hashish’ then these people continued to use these drugs, when it is likely some people who answered yes were not continued users.  As a result, 57% of all participants were considered to be active users of marijuana, which seems unlikely.  In addition, the number of people included in this study (1213) was low for work such as this and so more likely to draw inaccurate conclusions.

“Since marijuana use is illegal in many jurisdictions, it is very hard to be certain about the health risks it poses.  However, although this paper has limitations there is enough evidence from other research to strongly suspect marijuana use increases the risk of some forms of heart disease, and it is certainly not harmless.”


Prof Wayne Hall, Director and Inaugural Chair, Centre for Youth Substance Abuse Research, University of Queensland, said:

“This is an interesting paper despite its limitations that are clearly acknowledged by the authors, namely, the crude binary measure of cannabis use (yes/no), the assumption that cannabis users continued to use throughout the follow up, and the relative youth of the cohort which means that they have only passed through part of the risk period for developing cardiovascular disease.

“All of these limitations would reduce rather than increase the chances of finding any effect of cannabis use on cardiovascular disease.  This makes it all the more surprising that the authors found an elevated risk of high blood pressure in cannabis users.  The finding requires replication but it is not implausible because of other evidence which points to a possible increase in cardiovascular disease among cannabis smokers, especially those who do so regularly (e.g. weekly or more often) over periods of months or years.”



* ‘Effect of marijuana use on cardiovascular and cerebrovascular mortality: A study using the National Health and Nutrition Examination Survey linked mortality file’ by Barbara A Yankey et al. will be published in the European Journal of Preventive Cardiology at 07:30 UK time on Wednesday 9 August 2017, which is also when the embargo will lift. 


Declared interests

Dr Graham Wheeler: “I am employed by UCL, have a visiting researcher position at the MRC Biostatistics Unit at the University of Cambridge, am a Fellow of the Royal Statistical Society, and a voluntary research committee member for Chiltern Music Therapy, a not-for-profit organisation providing music therapy services.  I have no financial conflicts of interest.”

Dr Amitava Banerjee: “I have no conflicts of interest.”

Ian Hamilton: “I am affiliated to Alcohol Research UK.”

Dr Tim Chico: “I am a committee member and Treasurer of the British Atherosclerosis Society, a charity established in 1999 to promote UK atherosclerosis research.”

Prof Wayne Hall: “No competing interests to declare.”

None others received.

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