Legalized marijuana will soon be a reality in Canada. This opening up of legal access will have a variety of health implications. In addition, marijuana is increasingly prescribed for much of what ails us, from the pain and inflammation of osteoarthritis to a host of other conditions, many of which affect seniors – the same people who are at greatest risk of heart disease. What the legal use of marijuana could mean for your heart, whether you’ve already had a cardiac event or are at risk of having one, is in many ways an open question.
Physician and researcher Andrew Pipe, MD, and behavioural scientist Robert Reid, PhD, both of the Ottawa Heart Institute’s Division of Prevention and Rehabilitation, are widely published experts in the prevention of heart disease. As Dr. Pipe put it, “When we consider marijuana and issues relating to the heart we really are steering into terra incognita.”
The long-standing illegal status of marijuana has made it a lower priority for health researchers and posed a hurdle to conducting studies. What we do know about the drug’s effects on the cardiovascular system is not a lot, said both men.
“We’ll see an increased interest in research as a result of the liberalized approach to marijuana,” added Dr. Reid, Deputy Chief of the Division.
What the Research Tells Us
A 2014 review article in the New England Journal of Medicine summarized what is known about the effects of marijuana on all aspects of health. The authors found that marijuana use has been associated with vascular conditions that increase the risk of heart attack and stroke, although the mechanisms by which that happens aren’t clear. The article also noted that the risk is mainly associated with immediate use of marijuana and isn’t necessarily cumulative.
There is some epidemiological evidence that marijuana use in young men, in particular, is associated with an increased risk of heart attack within an hour of use, said Dr. Reid, whereas the risk in young women is not increased to the same degree.
Another article, from 2013 in the American Heart Journal, found that habitual marijuana use before a heart attack was associated with a higher mortality rate over the next 18 years, although the difference wasn’t statistically significant.
Both Drs. Pipe and Reid expect increased interest in research around marijuana use. They also both said that, based on what evidence there is, as well as common sense, trying this newly legal drug should be approached with caution and that, if you must, then don’t smoke it.
Physiological Effects of Smoking Marijuana
When you smoke marijuana, your heart rate and blood pressure increase. At the same time, smoking in general, whether tobacco, marijuana or a mixture of the two, reduces the capacity of the blood to deliver oxygen throughout the body – an effect of the products of combustion such as carbon monoxide. This combination adds up to increased risk for a cardiac event like a heart attack, particularly while a person is smoking and immediately afterward.
“There is a strong case for saying no one should ever smoke marijuana because of the products of combustion,” said Dr. Pipe. “We can say with absolute certainty that smoking marijuana will harm the ability to deliver oxygen, and that’s not good.”
While Dr. Reid pointed out that the relationship between marijuana use and heart disease remains, at this point, “largely theoretical,” there is existing evidence that marijuana use may lead to quicker onset of exercise-induced angina during a stress test among people with heart disease. This suggests another reason why it is not a good idea to use marijuana, he noted. He also noted that marijuana use could be problematic for people with an irregular heartbeat, or arrhythmia, because it activates the sympathetic nervous system.
What Happens to Your Heart When You Use Marijuana?
- Your heart rate and blood pressure increase, forcing your heart to work harder.
- If you smoke the marijuana, the capacity of your blood to transport oxygen throughout your body, including to your heart, is reduced.
- The result is strain on your heart and a reduced ability to handle increased demands.
The Problem with Assessing Impact
In theory, it shouldn’t be that difficult to assess the impact of marijuana use. In reality, though, it’s actually difficult to untangle.
For one thing, marijuana is rarely used in isolation. Generally, tobacco, alcohol or both are involved. And while the amount smoked is less with marijuana, experience has shown that smoking marijuana can make it harder to quit smoking tobacco. In reality, most people use the two together, thus, as Dr. Reid punned, “clouding the issue.”
“Significant numbers of individuals who smoke lots of marijuana daily come to us because they want to stop smoking,” said Dr. Pipe. “But for heavy marijuana smokers mixing it with tobacco, the likelihood of quitting is much less.”
Another problem relates to dose control. There are no standards for the production of marijuana and the amount of THC (the ingredient that gives marijuana its “high”) it contains. The problem becomes more acute when marijuana is eaten, said Dr. Pipe. People who eat marijuana may not feel the effect as quickly as those who smoke it, leading them to use more.
Marijuana and Depression
One concern frequently raised is the link between marijuana and depression. Depression is a known risk factor for heart disease, in part because it interferes with a patient’s ability to adopt healthier behaviours, and many believe that marijuana use can lead to depression.
But it’s a chicken-and-egg kind of argument. We really don’t know whether marijuana causes depression, said Dr. Pipe, or whether people who are depressed use marijuana at least in part as an effort to self-medicate. Either way, it underscores yet another area where more research is needed.
Eating marijuana is an option to avoid the toxins associated with smoking it, but other risks are likely similar.
“Edible forms would be different in terms of impacts on the oxygen carrying capacity of blood,” said Dr. Reid, “But there would still be some [effects] in terms of heart rate and blood pressure.”
This is also a rich area for future research, said Dr. Pipe, particularly isolating and purifying the chemical compounds in marijuana that have a therapeutic effect while eliminating the psychoactive ingredients.
“We don’t advise people in pain to chew willow bark [the source of the active ingredient in Aspirin], we tell them to take Aspirin,” he said. “There’s no reason marijuana should be any different.”
What’s a Patient to Do?
With all this uncertainty, it’s hard to know the wisest course for a patient. Dr. Reid advised that patients “start asking” their care providers about using marijuana just as they would about alcohol and to use caution, until something more definitive is known.
“[Smoking marijuana] probably wouldn’t be a good idea for someone with ischemic heart disease,” said Dr. Reid, because of the reduced oxygen transport through the body. “There’s no reason to begin marijuana use, and there could be more risk associated with it.”
“Don’t smoke it!” added Dr. Pipe. “Have a careful, considered conversation with the physician who is responsible for your care.”
“I’m pretty confident saying if you don’t use it now, there’s no reason to start,” concluded Dr. Reid.
A previous version of this article was originally published in July 2017.