Heroin is a drug, and belongs to the group of drugs called narcotics - these can relieve pain and make you go to sleep. Medicines are sometimes narcotics, such as codeine and morphine, in order to relieve pain. Heroin is made from the juice of an opium poppy. However, while codeine and morphine can be used in medicine, heroin is not usually because to some people it is very addictive.
In most countries, heroin is illegal. However, some people still take heroin - mainly because it gives a feeling of euphoria. Many people become addicted to heroin, and need to take it more and more. Withdrawal syptoms can happen when heroin is not available, and they will go to great lengths to get heroin; sometimes leaving their families or resorting to crime in order to obtain this drug. Heroin becomes the most important thing in their lives.
If someone injects heroin directly into their veins with an unsterilised needle, many pathogens may enter the body. For example, many people are infected with HIV when sharing needles.