Endorphin
Etymology
A portmanteau of "endogenous" (produced within the body) and "morphine," coined in the 1970s when researchers found the brain makes its own morphine-like painkillers. The name literally means "the morphine within."
noun · /en-DOR-fin/
A natural chemical the body releases to dull pain and lift mood.
What it names
Use it for the body’s homemade painkiller — the surge behind a "runner’s high" or relief after injury.
Example
The endorphins from the long run left him aching but strangely elated.
Related idea
Endorphins act on the same receptors as opioid drugs like morphine — which is exactly why those drugs are so powerful and addictive: they flood a system the body meant to dose sparingly. A hormone signals organs; an endorphin mainly modulates pain and pleasure.