Heroin, and all opiates, have a chemical structure similar to endorphins, a class of chemicals present in the brain. Endorphins are naturally manufactured in the brain to provide relief when the body experiences pain or stress.
Endorphins flood the synapses (the gaps between neurons) in the brain. They usually inhibit neurons from firing, and produce relief from pain and even euphoria. Endorphin levels go up when a person exercises, goes into labor, or has a high level of stress.
When someone takes morphine or heroin, the morphine molecule binds to the endorphin-receptor sites on the neurons, and mimics the function of natural endorphins. However, the user can control how much of the false "endorphin" her or she receives. Humans are pleasure-seeking organisms, so the motivation to self-administer such a drug is overpowering. Over time, the person cannot do without the drug – he’s addicted.
When someone takes heroin, he or she feels a sudden rush of pleasure that lasts for a minute or two, while the heroin bathes the brain. Soon the heroin is distributed by the bloodstream and changed into a more useable form of morphine. After the rush, the high lasts for four or five hours and is caused by the morphine diffusing from the bloodstream into the brain. It is described as a warm, drowsy, cozy state. Addicts report a profound sense of satisfaction, mild dizziness, and a sense of 'distancing' or apathy toward whatever is going on in the environment.
After a period of use, many addicts no longer experience the euphoria. They use heroin only for relief of painful withdrawal symptoms.