What Cannabis Smoke Does to Your Body: Heart, Lungs and Vision

Smoking weed can affect your body beyond the high: a large Stanford study linked smoking more than once a month to a higher risk of heart attack, especially before age 50, and some studies suggest cannabis smoke can harm blood vessels much like tobacco smoke, for smokers and bystanders alike. It can also irritate your lungs and temporarily blur your vision. Lots of people smoke cannabis today, whether to calm their nerves, to steer clear of tobacco or just to hang out, and as its popularity grew, so did research on a plant that does have plenty of real upsides.  

At the same time, myths about cannabis started making the rounds. One of them was that sharing a room with someone smoking weed has no health consequences. That one doesn't hold up. 

According to several studies, weed smoke can be as harmful as tobacco smoke for heart function and circulation, and it affects both smokers and the people around them. So what health problems can weed actually cause?

What are the health effects of smoking weed?

A study by researchers at Stanford University found a strong association between frequent cannabis use and a higher risk of early heart attack and cardiovascular disease. The results were published in the scientific journal Cell. 

The research also found that tetrahydrocannabinol (THC), the main psychoactive compound in the plant, inflamed the cells lining the inside of blood vessels and promoted atherosclerosis (hardening or thickening of the arteries) in lab tests on mice.

For the study, researchers analyzed records from 500,000 people aged 40 to 69, drawn from the UK Biobank. Nearly 35,000 participants said they smoked cannabis; of those, about 11,000 smoked more than once a month.

The researchers reported that people in the more-than-once-a-month group were more likely than others to have a heart attack, even after controlling for factors like age, body mass index and sex. They were also more likely than non-users to have their first heart attack before age 50.

Why does this happen?

When you smoke weed, THC binds to a receptor called CB1 on cells in the brain, the heart and the vascular system. This receptor normally responds to the body's own cannabinoids, called endocannabinoids, which help regulate mood, pain perception, immune function and metabolism. But frequent cannabis use can overactivate CB1, which may lead to inflammation and atherosclerosis. That kind of overactivation is also associated with obesity, cancer and diabetes.

That's why researchers are working on molecules called antagonists that block CB1 when the receptor is overactive, as in obesity. 

According to Spanish newspaper El Economista, the study's authors then searched a large database of protein structures for molecules similar to CB1 antagonists that could block THC's inflammatory and atherosclerotic effects without the psychiatric side effects. 

They found a small molecule in soybeans and fava beans called genistein that could block weed-induced inflammation and atherosclerosis. And because genistein can't cross the blood-brain barrier, it doesn't cancel out THC's other effects (stimulating appetite, easing pain and blocking nausea), which are exactly what medical cannabis patients are after.

When they added genistein to human endothelial cells treated with THC, or to mice with high cholesterol injected with THC, they found that it blocked weed's cardiovascular effects without any psychoactive effects of its own.

Still, there's a lot left to learn about this molecule. Joseph Wu, the study's senior author and director of the Stanford Cardiovascular Institute, pointed out that more and more people see marijuana as harmless or even beneficial. He also noted that while cannabis clearly has important medical uses, recreational users should think carefully before overdoing it.

How harmful is cannabis smoke?

According to several studies, weed smoke may be as harmful as tobacco smoke for heart function and circulation, and it affects both smokers and secondhand smokers. A Stanford study found a strong association between frequent cannabis use and a higher risk of early heart attack and cardiovascular disease. On top of that, frequent use can overactivate a receptor called CB1 on cells in the brain, heart and vascular system, which may cause inflammation and atherosclerosis, conditions associated with obesity, cancer and diabetes.

Which is worse: cigarette smoke or joint smoke?

Smoking a cigarette is generally considered more harmful than smoking a joint, mostly because of how much and how often people smoke them. Tobacco contains a long list of toxic, cancer-causing chemicals that are extremely hard on the lungs and other organs. Joint smoke contains some of the same substances, but typical cannabis use is less frequent and lower in volume than tobacco use, which lowers the risk of long-term lung damage. Still, any kind of smoking can be bad for your overall health and can affect your breathing and other aspects of physical and mental health.

How does smoking weed affect your lungs?

Smoking joints comes with some consequences: it can irritate and inflame your lungs, which can lead to chronic respiratory problems and reduced lung capacity. Weed smoke also contains many of the same toxic chemicals as tobacco smoke, which raises the risk of lung problems and cancer.

What health problems can weed cause?

Smoking weed can be rough on your lungs, causing airway irritation, lung inflammation and reduced lung capacity. Cannabis smokers can also develop symptoms similar to chronic bronchitis, like coughing, phlegm and wheezing. And weed smoke can carry toxic and carcinogenic substances that may increase the risk of lung disease and cancer. Keep in mind that how you consume cannabis (smoking, vaping, eating) changes the risks and benefits for your lungs.

Other effects: your eyes and vision

A team of researchers at the University of Granada in Spain found that smoking cannabis also significantly impairs vision, even though more than 90% of users believe it has no effect on their eyesight, or only a slight one, as reported by Argentine news site Filo.News. 

One of the best-known effects of smoking weed is red eyes. That happens because THC lowers blood pressure, which dilates small blood vessels and increases blood flow throughout the body. That, in turn, makes the blood vessels in your eyes expand, causing redness. However, weed's impact on peripheral vision, eye pressure and visual processing still isn't fully understood. 

The study, published in the Nature journal Scientific Reports, set out to measure the effects on vision and compare them with what users perceived. The team ran vision tests on 31 cannabis users, both sober and while high, and also asked participants how they felt their vision had changed.

They found that after smoking, several aspects of vision got significantly worse: visual acuity, contrast sensitivity, depth perception (stereopsis), the ability to focus and sensitivity to glare. Even so, 30% said their vision hadn't changed at all, and 65% said it had gotten only slightly worse.

There's still a lot to learn about the plant. But the data already shows that smoke, as we know, is bad for your health, and many of its consequences come down to users not being aware of them. So while there's plenty left to discover, the advice is simple: be careful and be mindful about how much you smoke.