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Every summer, pot-loving athletes head to marijuana hotspots like Denver, Colorado, to get high and exercise.

They’re participants in The 420 Games, a roving, Olympic-ish event complete with a 4.20-mile race, kickball tournament, and, according to the event website, social events that “break down the stereotypes that have been built up during the era of cannabis prohibition.”

The events are part of a growing trend of making marijuana a regular part of an active lifestyle. For all the excitement they generate, Brook Henry, a cannabis researcher at the University of California, San Diego, tells Inverse these events are lacking one crucial thing: scientific evidence.

See also: “This Is Your Brain on Drugs”

What We Don’t Know

Anecdotally, a lot of people love the experience of exercising while high on marijuana — and on plenty of other drugs, too. Case in point: There are dozens of Reddit threads dedicated to discussing the best drug/workout combo, but actual research on the intersection of marijuana and physical exercise is scant.

That’s unfortunate because marijuana is widely used and incredibly complicated. Unlike other drugs, marijuana isn’t easily categorized as a depressant, stimulant, or hallucinogen. Rather, it seems to flit between categories, with users all responding drastically different to the drug, thanks in part to an “entourage effect,” where dozens of cannabinoids like THC and CBD and pungent oils called terpenes work together to give you that special feeling.

In the lab, researchers can isolate each of these chemicals and test their individual effects, but it’s not exactly true to life, where the drugs are working in tandem. We know, for example, how CBD responds to a single receptor in a petri dish, but in the body it’s actually interacting with 60 different receptors. Terpene linalool, meanwhile, is associated with sleepiness in the lab, but, perplexingly, it’s a key component of more stimulating strains of marijuana.

What We Do Know

So when someone takes all this chemical complexity into the gym, it’s hard to say what will happen. They may feel great, they may feel terrible, or they may feel nothing at all. Still, Henry says, two things are pretty certain. While one’s cool, the other’s troubling.

First, marijuana really does seem to alleviate the aches that tend to accompany exercise. “We do know that marijuana can have effects reducing inflammation, reducing pain, and potentially improving affect,” Henry says. That’s part of the reason why doctors prescribe medicinal marijuana, which increases appetite and minimizes pain in the sick. The thing is, exercise also releases your body’s homemade mood-boosters, like endorphins and endocannabinoids, calling into question the value of that pre-gym bong hit. “There are anecdotal reports that athletes use marijuana to improve their mood and their performance,” too, Henry says, “but there is no research.”

Unfortunately, these potentially positive side effects may be undercut; marijuana can alter your cardiovascular system, Henry says, sometimes in really dangerous ways. In 2001, researchers reported in the journal Circulation that among individuals who had already had a heart attack, the risk of a second cardiac event increased almost 5 times in the 60 minutes after using marijuana.

While the study is far from definitive, marijuana has long been known to have a short-term effect on your heart. More specifically, Henry says, it can elevate your heart rate and send your blood pressure bubbling up and shooting down. Even with a healthy heart, artificially raising your heart rate by smoking pot before you hit the gym is likely to tucker you out. As a result, you may experience slightly slower reaction times or give up on your workout early. Meanwhile, if your blood pressure’s dropping, that can make you feel lightheaded or make you faint, both of which are frowned upon in the gym.

Unfortunately, until scientists set their sights on the mysteries surrounding marijuana and exercise, things won’t ever be clear. While Henry sees the potential for both pros and cons to be revealed as the science develops, for now he advises gym-goers to be careful. “There are a lot of unknowns out there,” he says. “I’d be very cautious, in general, about taking cannabis and exercise.”


See also: “Marijuana Strain Descriptions Are Bunk, Chemist Says”

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