What actually causes the 'runner's high' and is it really like cannabis?

The answer

For decades, exercise-induced euphoria was attributed to endorphins, but a paradigm shift occurred when researchers discovered that endorphins are too large to cross the blood-brain barrier. A 2015 study in PNAS by Johannes Fuss at the University of Hamburg demonstrated that the runner's high is primarily caused by endocannabinoids - specifically anandamide (from the Sanskrit word 'ananda' meaning bliss) - which are lipid-soluble and freely enter the brain. Mice bred without cannabinoid receptors showed no anxiety reduction after running, while those lacking opioid receptors still experienced the runner's high. Anandamide levels increase by approximately 30% after sustained moderate exercise. These endocannabinoids bind the same CB1 receptors as THC in cannabis, explaining the euphoria, reduced anxiety, and altered time perception runners describe.