The feel-good chemicals commonly associated with exercise and a runner’s high are called endorphins.
Endorphins are naturally produced opioid neuropeptides that bind to opioid receptors in the nervous system. They can reduce the perception of pain and contribute to feelings of well-being. Exercise has long been associated with increased endorphin activity, which helped popularize the explanation that these chemicals create the relaxed or euphoric feeling some people experience after prolonged running.
The biology is more nuanced than the popular slogan suggests. Endorphins are relatively large molecules and do not readily cross the blood–brain barrier, so researchers now think they may not fully explain the mood elevation of a runner’s high. Endocannabinoids, including anandamide, appear to play an important role because they can act within the brain.
That nuance does not make “endorphins” an incorrect quiz answer: it remains the standard public-facing term for exercise-related pain relief and good feelings. Not everyone experiences a runner’s high, and its intensity can vary with exercise duration, intensity, fitness, and individual biology.