When someone injects heroin, it alters chemicals in the brain resulting in an intense "rush" as the drug reaches the brain quickly. The following high effect can be described as a strong feeling of contentment, euphoria and relaxation. It may also produce some dizziness and drowsiness, although users describe these feelings as also enjoyable. Heroin also has anaesthetic effects resulting in a numbing feeling. The rush lasts about 2 minutes, with the high lasting for about four to five hours. 

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It's this feeling, which users can quickly become addicted to, and unlike alcohol and other drugs, there generally isn't a hangover or comedown from initial use, which hooks in new users. However, tolerance builds quickly and they need more and more of the drug to get the same feeling.,

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Eventually, the brain cannot produce natural amounts of dopamine on its own and the user needs heroin to just feel normal. The numbing effect can also hide many physcial and mental symptoms. Increased use, increase the risk of fatal over-dosing.

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Heroin is one of the most addictive drugs, and withdrawal symptoms become so painful, it's even more motivation to continue using the drug.

 

Addiction means people continue to seek out and use heroin, even when it is causing catastrophic consequences in many areas of their lives, including mental and physical health, relationships, ability to work and meet responsibilities.