Scientists have long assumed that the runner's high phenomenon was created by the meeting of endorphins with the body's opiate receptors, which are what exogenous opioids like morphine and heroin bind with.
As it turns out, runner's high is actually caused by the binding of endocannabinoids to the body's cannabinoid receptors, much like THC does after a hit of weed. What's more, the body's endocannabinoid system plays a central role sexual arousal.
Researchers investigated two core features of the runner's high, namely euphoria and anxiety levels. A pool of over 60 experienced runners were studied, some of which were given a drug that blocks the endorphin uptake.
After performing a 45-minute run to achieve the runners high, scientists found that nearly all of the subjects' were content and relaxed, regardless of whether their endorphins were blocked.
Scientists were already skeptical of endorphins' contribution to the runner's high, because unlike endocannabinoids, the hormones can't make the leap from bloodstream to the brain.
This study provides evidence to support the conclusion that endocannabinoids—not endorphins—are responsible for the runner's high.