I need to know what's the effect that heroin gives a person that shoots it up?

High-dose endorphin: Human beings (and other animals as well) make their own "opioids" during times of stress, when in pain, when exercising and when happy - these are enkephalins and endorphins. Large amounts of these cause an intense euphoria - a "rush" or "high." Heroin is an extremely effective drug that crosses into the brain very quickly and has fewer side effects than morphine, producing this same euphoria.