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In our new, bi-weekly series CUriosity, experts across the �鶹��Ѱ�����Boulder campus answer pressing questions about humans, our planet and the universe beyond. 

This week, Psychology and Neuroscience Professor Angela Bryan, answers: “What causes the runner’s high?” (Hint: It may not be what you think.)

Runners cross the Verrazano Narrows Bridge in New York City. More than 51,000 runners will line up for this year's New York Marathon November 3. (Credit: via )

It can set in after just a few miles: Nagging aches grow numb, sense of time slips away, colors brighten and thoughts grow crisp. Afterward, runners say they feel less anxious, stronger and more confident—even euphoric.

 

   Previously in CUriosity

Why does playing in the dirt feel so good?

But what causes this legendary “runner’s high?”

For decades, scientists have suspected that endorphins and other “endogenous opioids”—pain killers made by our own bodies—are at play, coursing through our blood stream when we work out and attaching to the same receptors in the brain that drugs like morphine or oxycodone do. 

But, Bryan says, running may actually be more like smoking a joint than taking a pain pill. 

“For a long time, everybody thought that the runner’s high was caused by the activation of the endogenous opiate system, but we are now beginning to think it is actually the endogenous cannabinoid system instead,” she explains.

She would know: The psychologist often starts the morning with a run, then heads into her lab where she studies how cannabis, a.k.a. weed, affects the human body.

Cannabinoids are essentially the active ingredients in cannabis. Some “exogenous” (from outside the body) cannabinoids, like THC (tetrahydrocannabinol), promote intoxication. Others, like CBD (cannabidiol) quell inflammation and anxiety. The human spinal column and brain are rich in cannabinoid receptors that these compounds attach to when someone eats a gummy or smokes cannabis flower.

But we also make our own.

Standing on the Boulder Creek path, her face glowing after a morning jog, Bryan explains:

“I feel amazing. It’s just this feeling of freedom and strength and confidence,” she says as a pack of runners shuffles by. “Everyone we are seeing out here today is having that same experience, and I would be willing to bet that none of them used cannabis before their run today.” 

When we exert ourselves, our tissues release cannabinoids which slip perfectly into those same receptors, reducing pain and sparking euphoria.

As Bryan notes in , long-distance runners often describe running as a “drug like” experience (complete with difficulty assessing the passage of time). They also experience something akin to withdrawal when they can’t run.

Numerous studies have shown that high-intensity exercise (running more so than walking) leads to higher circulating levels of endocannabinoids in the blood.

Meanwhile, studies have begun to debunk the endorphin-runner’s high connection.

One found that when opioid receptors were chemically blocked with a drug called naltrexone, athletes still experienced the runner’s high. Another found that when cyclists had higher circulating levels of endorphins, they actually felt more, not less, stressed out. 

While it’s not necessary to use cannabis to get the runner’s high, many athletes, particularly ultra long-distance runners, do mix weed and workouts. They report that supplementing the cannabinoids their bodies make with ones from a dispensary can enhance the high or bring it on sooner. 

But there are downsides to that.

In one study of 42 runners published in the journal Sports Medicine, Bryan found that using THC or CBD during exercise did indeed boost motivation and make exercise more fun. But it also slowed folks down.

Her advice for those gunning for a fast 5k or a marathon PR: Stick with the high your own body gives you.