Heroin is an opioid - a very powerful painkiller. The body and the brain are packed with opioid receptors, meant for endorphines, the body's own natural pain-killing substances produced in emergency moments of shock or injury.

Heroin mimicks endorphines and binds rapidly with endorphine receptors, extending and magnifying their natural painkilling effect. The result is a surge of pleasurable sensation, or "rush." This rush is usually accompanied by a warm feeling and a sense of well-being.

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rapid effect
The effects are rapid but depend on method on ingestion. Intravenous injection provides the greatest intensity and most rapid onset of euphoria (7 to 8 seconds).

Intra-muscular injection produces a relatively slow onset (between 5 to 8 minutes). Sniffing or smoking usually provides peak effects within 10 to 15 minutes.

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First-time or infrequent users often experience nausea and vomiting.

Once the initial rush has passed there can be a feeling of heaviness, as though your bones have melted down into your feet; followed by a feeling of distance from events around you, as though you've been wrapped in cotton wool. The result is a comforting glow. Nothing matters.

afterwards
Heroin sedates the central nervous system, clouding mental function and making you feel drowsy for several hours after a dose. You may appear to be asleep, but actually be awake.