The most common way to take heroin is through injection. The rapid absorption of injected heroin into the bloodstream causes a rush and an intense high. At first, heroin users experience few lingering effects after a dose. The drug effects wear off after about six hours. Over time, however, addicts develop a tolerance to the drug's effects and require larger and larger doses of heroin to achieve the same high or any high at all. They become physically dependent on the drug and experience withdrawal symptoms when they are not using the drug.

Methadone, one of the most restricted drugs in the United States, acts as a substitute for heroin and relieves addicts of strong withdrawal symptoms.

The first...