Opioids can make you throw up—this can even happen to someone given opioids by a doctor—which is why many people don’t like taking them.

Your brain makes its own versions of opioids, called endogenous opioids. These chemicals act just like opioid drugs, attaching to opioid receptors in your brain. Endogenous opioids help your body control pain. If you’ve ever felt pleasantly relaxed after exercising a lot, that feeling was probably caused by the release of these natural chemicals (sometimes called “endorphins”) in your brain.