Ask a former IV heroin addict

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Much appreciated.

Because using heroin causes your brain to release massive amounts of dopamine into your system. After some time your body physically becomes dependent on that outside source of dopamine and stops producing the chemical on its own. So when you stop using heroin it takes a while for your body to start producing dopamine on its own again and up until your body balances out chemically, you experience that lack of dopamine as heroin withdrawal.

Interesting. I never would have thought that lack of dopamine makes you physically ill. Thanks for answering.

It's much more complicated than that, but dopamine is a driving factor.