Endorphins. Endorphins are natural opioids that help us tolerate the discomfort of exertion.22 The body’s own opioids are less strong than heroin, codeine, and morphine, but they too blunt pain and produce feelings of euphoria. Opioids allow us to go for a long hike or run without noticing our muscles are sore and our feet have blisters. They may also contribute to exercise addiction.
