This legal drug kills more Americans than heroin — here's what it does to your body and brain

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Despite being legal with a doctor's prescription, opioid painkillers can come with serious health risks.

The drugs belong to a larger class of drugs known as opioids, which includes legal, lab-produced drugs like oxycodone, fentanyl, and morphine as well as illegal drugs like heroin.

Since they slow breathing and act on the same brain systems as heroin, opioid painkillers carry serious risks, from overdose to, in rarer cases, addiction.

Read on to learn more:

Opioid painkillers capitalize on our body's natural pain-relief system.

We all have a series of naturally produced keys ("ligands") and keyholes ("receptors") that fit together to switch on our brain's natural reward system — it's the reason we feel good when we eat a good meal or have sex, for example.

But opioids mimic the natural keys in our brain — yes, we all have natural opioids! When they click in, we can feel an overwhelming sense of euphoria.

Opioid painkillers can have effects similar to heroin and morphine, especially when taken in ways other than prescribed by a doctor.

When prescription painkillers act on our brain's pleasure and reward centers, they can make us feel good. More importantly, though, they can work to reinforce behavior, which in some people can trigger a repeated desire to use.

You may also feel sleepy.

Opioids act on multiple brain regions, but when they go to work in the locus ceruleus, a brain region involved in alertness, they can make us sleepy. Why? The drugs essentially put the brakes on the production of a chemical called norepinephrine, which plays a role in arousal.