People smoke or inject heroin to get a calm feeling of relaxation. It is called "being high". The extreme happiness is known as euphoria. Heroin is a very addictive drug. This means that once a person starts taking heroin, they will want to take more and more. They will want to continue even when they know it is making them sick and harming them. Heroin is an illegal drug in many countries, unless it is used to treat a medical problem.