BY BEN THOMAS | Photo by Jeremy Lapak
Everyone who’s felt the joy of smiling, gasping for breath after a strenuous workout, knows that physical exertion often leads to a rush of delight. Whether you run, swim, kickbox, or lift weights, you’ve probably experienced the feeling known as “runner’s high”—a flood of positive vibes just as powerful as the body buzz from a high-grade indica strain.
You’ve probably also heard about endorphins, the chemicals (supposedly) behind this buzz. But as recently as this year, some intriguing scientific results have called the role of endorphins into question. The true source of runner’s high, these results suggest, may be a completely different class of organic molecules: endocannabinoids—your body’s very own equivalents of THC.
This exciting discovery suggests that runner’s high may be even truer to its name than anyone guessed. Here, we’ll take a closer look at this phenomenon, and see whether endorphins or endocannabinoids—or both—are truly responsible for post-workout euphoria.
Endorphins: the so-called "happy chemicals"
Endorphins became famous in the 1980s, when a series of studies demonstrated that these naturally produced chemicals act on the same nerve receptors that respond to opiates like morphine and heroin. Unlike those synthetic drugs, however, endorphins flood into our bloodstreams when we laugh, meditate, have sex, or just sit in the sunshine. Strangely enough, scientists also found that our bodies crank out one particular class of endorphins, known as beta-endorphins, in response to painful sensations, like menstruating, getting a tattoo, or eating a super-spicy curry. This discovery led to the logical conclusion that endorphins play a crucial role in pain management—a fact that researchers soon verified.
But why stop at just pain management? Since we know we release endorphins when we’re feeling pleasure, and since endorphins activate the same neural receptors as opiate drugs, scientists reasoned that endorphins must be our bodies’ “happy chemicals”—not just reducing pain, but actively creating feelings of physical pleasure, as in the case of runner’s high.
That claim was widely accepted as scientific fact for more than 30 years—until 2015, when a surprising new discovery showed it was not only inaccurate, but actually impossible.
A likelier set of candidates: endocannabinoids
Even as endorphins received all the glory, scientists pointed out one very inconvenient fact: endorphin molecules are too large to pass from the bloodstream into the brain. This means that, while endorphins can (and do) suppress pain in the peripheral nervous system, there’s no physical way they can directly trigger emotions of pleasure.
What scientists needed, then, was a different chemical mechanism to explain the sensation of runner’s high. Endocannabinoids, the body’s internal analogues of THC, looked to be the likeliest candidates. In a 2012 study published in the Journal of Experimental Biology, a team led by David A. Raichlen found that ferrets don’t produce extra endocannabinoids when they sprint for short distances—unlike humans and dogs, who do pump out endocannabinoids during long-distance lopes.
While those results confirmed that endocannabinoids were central to runner’s high, they still didn’t rule out the possibility that endorphins were also involved somehow. To debunk that notion, Johannes Fuss—director of the Human Behavior Laboratory at the University Medical Center Hamburg-Eppendorf—knew he’d have to go one step further. In 2015, Fuss and his team at the University of Heidelberg chemically blocked endorphin receptors in mice, then set them running on little treadmills.
Sure enough, the anti-endorphin mice experienced post-workout bliss, including reduction in pain sensations—while, crucially, mice with blocked endocannabinoid receptors ended up exhausted and cranky after their workouts. Case closed, it seemed: endocannabinoids, not endorphins, are responsible for runner’s high.
Humanity's own high
Just one last objection remained: mice are not people—although their nervous systems are similar enough to ours that results obtained in mice often apply to humans, too. But in 2021, just to be sure, Fuss rounded up a group of experienced runners to repeat the experiment with human volunteers, using the drug naltrexone to block endorphin receptors in some people while using a placebo on others. (The drug they’d used to block endocannabinoid receptors in mice is illegal for use on humans, so the team had to adapt their approach accordingly.)
Once again, endocannabinoids proved their power. As in the mouse study, people with blocked endorphin receptors reported relaxation, euphoria, and pain reduction after a long run—and blood tests confirmed their nervous systems were flooded with endocannabinoids. Only a small fraction of volunteers on the placebo, by contrast, reported these post-run benefits.
In fact, the reason for runner’s high may lie deep in the mists of our evolutionary past. “When the open savannas stretched and forests retreated,” Fuss told The New York Times, "it became necessary for humans to hunt wild animals by long-distance running. Under such circumstances, it is beneficial to be euphoric during running.” Perhaps, then, it was our distant stone-age ancestors who first discovered the joy of—quite literally—getting high on their own supply.
Ben Thomas is a journalist and novelist who's lived in 40+ countries. He specializes in telling stories from the frontiers of science, history, culture and the cosmos—and the points where all these fields intersect.