Does the 'runner's high' really come from endorphins?

Probably only in small part. Endorphins, whose name is a contraction of 'endogenous morphine', really do block pain and boost well-being, and the body releases them during exercise. But when researchers experimentally blocked the opioid receptor that endorphins act on, exercise-induced euphoria could still develop: endorphins are not strictly necessary for it. The more likely culprits are endocannabinoids, the body's own cannabis-like molecules.

There is also a plumbing problem. More recent studies suggest endorphins play a limited role in the runner's high because they cannot cross the blood-brain barrier, the gate between the bloodstream and the brain. Endocannabinoids can. Endorphins may still help with the pain relief that exercise brings.

This fits other details of the phenomenon. The runner's high turns out to be a transient state of euphoria with reduced anxiety and a higher pain threshold, and it isn't even universal: it's a relatively rare experience, more tied to longer, sustained exertion than to short sprints, because endocannabinoid release is greater during longer, more extreme exertion. New runners rarely feel it at all, reporting discomfort instead; the positive mood changes are more common among regular runners than occasional ones.

Endorphins themselves have a well-documented history: they were identified piece by piece in the 1970s, after researchers in Aberdeen isolated related opioid peptides called enkephalins in 1973, and others went on to show that beta-endorphin, extracted from the pituitary gland, was even more potent than morphine itself. That makes them real and important, just probably not the star of the specific euphoric feeling that made them famous.