Heroin (diacetylmorphine or diamorphine) is a powerful narcotic drug which is derived from morphine. Despite its usefulness as a pain reliever, is no longer used in the United States, as a therapeutic drug. Its recreational use is generally prohibited or restricted in every country. Most westernized countries (except the US) allow for its use in palliative care situations.
Heroin was once an over-the-counter drug. It was even marketed as a children's cough remedy by Bayer, along with its other well known product, Aspirin. Heroin is actually diacetylmorphine, and is metabolised into morphine in the liver. This caused Bayer much embarrassment because they had marketed it as a cure for morphine addiction. The acetyl groups give it the ability to rapidly cross the blood-brain barrier, giving it a much stronger "rush" for abusers. This only increased its addictive properties and by the end of World War I it was banned in the United States, even by prescription.
People that are addicted to that drug, will be detoxed with the help of methadone. House used it to get rid of his leg pain in the episode The Softer Side.