When certain acupuncture points are stimulated your body releases opioid peptides, like endorphins. When you bisect the word endorphin you get endo - short for endogenous (originating within an organism) and orphin short for morphine. Endorphins play a role in your motivation, emotion, behavior, stress response, pain response, as well as controlling your food intake. Acupuncture has been proven to release endorphins which help to inhibit the sensation of pain and in some cases produce a feeling of euphoria similar to opioids.
So, without taking any medications, acupuncture can help your body produce its own natural morphine, allowing your body heal itself by reducing pain and stress.