The effect of cannabis on the heart, lungs and movement

Dr. Leafly's Nick Jikomes explains what we know about how THC affects physical performance.

In Part I we learned about the connection between physical activity and the endocannabinoid system. In general, physical activity leads to an increase in endogenous cannabinoids. Participation in rewarding activities, including voluntary physical activity, can increase the sensitivity of CB1 receptors in the brain, influencing how they subsequently respond to cannabinoids.

Cannabinoid receptors in the brain are crucial for motivation to exercise voluntarily, and the endocannabinoid system responds to physical activity. It is therefore plausible that plant cannabinoids such as THC, which stimulate the same CB1 receptors that respond to exercise, influence exercise performance.

Before we delve into the studied effects of THC on exercise performance in humans, let's take a quick look at the effects of THC on two tissue systems with high relevance to exercise: the lungs and the cardiovascular system.

Read Part I

Here's the euphoric chemistry behind your runner's high

Effects of marijuana smoke and THC on the lungs


Conclusion: No smoke is good for the lungs, but tobacco and marijuana smoke do not pose the same risks.

Inhaled smoke of any kind can damage lung tissue. This is not surprising: our lungs evolved to inhale/exhale atmospheric air, not the smoke produced by burning plant material. However, different types of smoke coming from different sources have different compositions. Therefore, they don't have exactly the same effects on the lungs. For example, there is some overlap in the composition of tobacco smoke and marijuana smoke, but also many differences between the two.

Both tobacco and marijuana smoke have been observed to cause various forms of cellular damage in lung tissue. Sometimes similar damage occurs caused by similar combustion byproducts from both types of smoke. There are also forms of harm seen with tobacco smoke that are not seen with marijuana smoke, and vice versa (specific examples are discussed here). This may be why there is currently no clear evidence that smoking marijuana is a risk factor for lung cancer.

A key difference between tobacco and marijuana smoke is that the latter contains cannabinoids (mainly THC). The lungs are home to both CB1 and CB2 receptors, key components of the endocannabinoid system. The CB1 receptor, which is responsible for the psychoactive effects of THC in the brain, is found in lung tissue in significantly higher concentrations than CB2. This means that THC can have direct effects on lung cells that are not seen with tobacco smoke. Cannabinoid receptors are also found in immune cells in the respiratory tract (mainly CB2 receptors), meaning cannabinoids like THC can affect lung inflammation.

Observed effects of THC or marijuana smoke on lung cells include abnormalities in alveolar macrophages (the most numerous immune cells in the lungs) from lung tissue of human marijuana smokers. This included a reduced ability to kill Staphylococcus aureus. Other experiments on animal tissue have generally found that THC impairs the immune response to lung pathogens.

Plant cannabinoids generally have an anti-inflammatory (immunosuppressive) effect. We typically think of anti-inflammatory effects as a good thing, especially because chronic inflammation is so common these days. But anti-inflammatory effects can be good or bad depending on the context. When the normal function of immune cells is compromised and their ability to respond to pathogens is impaired, that's a bad thing. When the body produces an excessive inflammatory response, suppressing it can be beneficial to your health.

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Modern humans suffer from chronic inflammation: more than half of all deaths are attributed to inflammation-related diseases. The high prevalence of chronic inflammation may even be a reason why cannabis use has increased among adults.

Asthma is an example of a long-term inflammatory lung disease that causes inflammation of the airways. It can be triggered by various causes. Smoke exposure generally worsens asthma symptoms. Theoretically, a cannabinoid with anti-inflammatory effects could help relieve the symptoms of an inflammatory disease such as asthma.

Although there are reports that THC can cause bronchodilation (enlargement of airways in the respiratory system), results have been mixed in studies of patients with breathing problems. For example, an early study found that nebulized THC caused significant bronchodilation in some asthma patients but the opposite (bronchoconstriction) in others. In a recent randomized control trial in patients with chronic obstructive pulmonary disease (COPD), no clinically significant effects of THC were found, either positive or negative.

According to a systematic literature review, short-term THC exposure is associated with bronchodilation, while long-term marijuana smoking is associated with increased respiratory symptoms consistent with obstructive pulmonary disease. Whether the bronchodilatory effects of inhaled THC could be beneficial for patients with inflammatory lung disease is unclear.

Effects of THC on the cardiovascular system


Conclusion: People with existing cardiovascular problems should be more careful.

Given the prevalence of the endocannabinoid system in the body, it is not surprising that CB1 receptors are found on cells of the cardiovascular system, including the blood vessels and heart. In general, THC acts as a vasodilator through CB1 receptors, relaxing blood vessels. This is why people's eyes often turn red when they're stoned – the blood vessels in the eye have relaxed, leading to increased blood flow.

Inhaling THC in the form of smoke or vapor causes an acute (short-term) increase in blood pressure and tachycardia (increased heart rate). For the average healthy person, this isn't necessarily a bad thing. For example, there is an increase in heart rate and blood pressure in response to exercise because the body's tissues have a greater need for oxygen and nutrients. People with pre-existing cardiovascular problems should be more careful.

The biphasic effect is known for the psychoactive effects of THC: low vs. high doses can produce different effects. Biphasic effects may also play a role in THC's influence on the cardiovascular system: lower doses tend to increase blood pressure and heart rate, but bradycardia (slow heart rate) and hypotension (lower blood pressure) have been observed at higher doses.

In addition to the direct influence of THC on the cardiovascular system via CB1 receptors, smoking in particular is likely to have an impact on oxygen requirements. Smoking leads to higher levels of carboxyhemoglobin in the blood and reduces oxygen supply. These types of cardiovascular changes are expected to influence exercise performance, although, as we will see below, research on this is limited.

Exercise performance in chronic cannabis users compared to non-users

Conclusion: Chronic smokers stay fit.

A handful of studies have examined various aspects of exercise performance in healthy chronic cannabis users and compared them to healthy non-users. In these studies, cannabis users were asked to abstain from consumption before testing (i.e., they were sober at the time of measurement). These studies have generally found no difference, positive or negative, between chronic cannabis users and non-users in the following fitness metrics:

  • VO2max (the maximum rate of oxygen consumption during physical exertion, a measure of aerobic fitness)
  • Blood pressure
  • Muscle strength and endurance measures
  • Perceived exertion

Again, these things were found to be no different between non-users and chronic users tested in the fasted state. Whether these metrics would have been different for chronic cannabis users if they had exercised while drunk has not been investigated.

Although research is limited, there is currently little evidence that chronic cannabis use, when used outside of a training or competition context, has a significant impact on basic measures of physical performance in non-elite recreational athletes.

Effects of acute cannabis intoxication on exercise performance

Conclusion: Top performers still performed well.

Very few studies have been conducted looking at the acute effects of THC intoxication on exercise performance. I found only one human study that looked at healthy participants training to peak performance. In this small study, participants performed increasingly demanding workloads while fasting until they reached leg failure, compared to 10 minutes after smoking a joint. No differences were generally observed at maximal exercise (measures included VO2, VCO2, heart rate, and ventilation), although the expected physiological effects of THC intoxication (e.g., tachycardia, increased resting heart rate, and increased blood pressure) were observed.

Aside from such small studies, very little work has been done on the acute effects of THC intoxication on exercise performance in healthy adults. While the few studies available have largely found little to no significant effects of acute THC consumption on exercise performance, there is not enough well-controlled scientific research to draw firm conclusions. Given THC's known physiological effects on the cardiovascular system, it would be surprising if THC had no effect on performance. To be sure, larger, better controlled studies need to be done.

Because cannabinoids like THC are fat-soluble molecules, they can accumulate in body fat and slowly leak out over time. This suggests that the amount of body fat you have and the rate at which you burn it through exercise could affect THC levels in the blood. This will be the subject of Part III.

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