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The best detail in Christian Cheung’s in the Journal of Applied Physiology is the rigorous, screening the subjects had to undergo: a urine test to prove that they did, in fact, use drugs at least once a week. Such is the world of cannabis research, which still tends to make funding agencies and ethics committees a little uncomfortable. Cheung’s subjects, fortunately, all had plenty of experience and got the green light to head into the lab and test their endurance while high.

As marijuana laws have been loosened in jurisdictions around the world, there’s been lots of discussion and debate about whether it’s a performance-enhancer, particularly after high-profile controversies like sprinter and suspension at the Olympic Trials in 2021. In endurance sports like ultrarunning, it has a long history: “The person who is going to win an ultra is someone who can manage their pain, not puke, and stay calm,” ultrarunner back in 2015. “Pot does all three of those things.” Based on their prior self-experimentation, many of Cheung’s subjects expected to see a boost in performance. But that’s not what the data showed.

As a grad student in the University of Guelph’s , Cheung and his supervisor, Jamie Burr, have conducted a series of studies on the links between cannabis, health, and athletic performance. A , for example, found that young habitual pot smokers had altered heart function and stiffer arteries than non-smokers despite being apparently healthy. They also published in 2021 summing up what’s known about the links between cannabis and sports-related outcomes like performance, pain, sleep, and recovery. The overall answer: not much.

The new study compared the effects of THC, the psychoactive ingredient in cannabis (i.e. the one that makes you high); and CBD, the other main molecule in cannabis, which has become popular for recovery but is generally not considered psychoactive. Subjects did a 10-minute warm-up at a constant pace followed by a 20-minute all-out time trial, under four different conditions: control, smoking THC-predominant cannabis (S-THC), vaping THC-predominant cannabis (V-THC), or vaping CBD-predominant cannabis (V-CBD). The 14 subjects, drawn in part from local cycling and rowing clubs, were mainly recreational athletes.

The main result (as Cheung explains in ) is that THC slowed them down, while CBD had no effect. Here’s the average power output for the four time trials:

Those differences are pretty substantial: a drop of 5.1 percent for smoking THC and 7.5 percent for vaping it. The bigger effect for vaping is consistent with research suggesting that vaping produces bigger and faster pharmacological effects than smoking, but they didn’t measure cannabinoid levels in the blood, so it’s hard to be sure about the respective doses. (They did standardize the inhalation protocol: repeated bouts of five-second inhalation then 10-second hold, with 45 seconds of recovery. Deviations from this protocol were permitted as needed for “coughing or laughing.”) Interestingly, the results contrast with an earlier study that found no effect of edibles on time-trial performance, suggesting that the negative effects are triggered in the airways.

The trickier part is figuring out why the subjects slowed down on THC. Cheung and his colleagues measured a bunch of physiological and perceptual parameters, but the results are puzzling. During the submaximal warm-up, THC (but not CBD) raised heart rate by between 14 and 18 beats per minute, which is a big difference. But during the all-out time trial, heart rates were identical. Other physiological parameters like oxygen consumption were also the same, although breathing was a little shallower in the THC trials. The subjects somehow managed to go slower on THC despite seemingly working their bodies just as hard.

The obvious alternative is that THC messed with their minds rather than their bodies. Maybe they just didn’t feel like trying as hard when they were high, or were too loopy to execute a good time trial. But their subjective ratings of perceived exertion were the same in all conditions. And their pacing patterns were similar too, so it’s not that being high messed with their ability to judge their effort and distribute their energy appropriately.

CBD, in this study, didn’t seem to have any effect on either the body or mind, which isn’t surprising since it’s billed as the non-psychoactive component of cannabis. But it’s interesting to compare these results to , this one from researchers at the University of Colorado Boulder, that had habitual cannabis users go for a run after smoking either a THC-dominant or CBD-dominant product. This one didn’t have a performance test, but focused on the subjective experience of exercising while high.

The that drew attention was that the runners reported a stronger runner’s high, the feeling of euphoria, effortlessness, and relaxation that (for some lucky people) sometimes accompanies a good run. The downside was that they also reported a greater sense of subjective effort, so cannabis seemed to accentuate both the positives and negatives. But the odd thing is that the CBD runs seemed to produce a greater accentuation of runner’s high and overall enjoyment than the THC runs—which is bizarre if CBD isn’t psychoactive. (The products used in the Colorado study were 24 percent THC and 1 percent CBD, or 1 percent THC and 20 percent CBD.)

So where does that leave us? “It’s still very early days in this type of research,” Burr told me. Cannabis’s classification as a restricted substance affects research in ways you might not expect: you’re not allowed to randomize subjects to different groups, for example, when you’re dealing with a Schedule 1 controlled substance. It seems increasingly likely that, in terms of its strictly physiological effects, cannabis doesn’t help and probably hurts endurance performance. How and why that happens remains an intriguing riddle. As for the psychological side—like Shelton’s claim that it could help performance in other, less direct ways—that’s an even trickier question. But it’s getting harder and harder to justify suspending athletes like Richardson for doing something with no apparent performance benefit.

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