Methadone (also known as Symoron, Dolophine, Amidone, Methadose, Physeptone, Heptadon, Phy and many other names) is a synthetic opioid, used medically as an analgesic and a maintenance anti-addictive for use in patients with opioid dependency. It was developed in Germany in 1937. Although chemically unlike morphine or heroin, methadone acts on the same opioid receptors as these drugs, and thus has many of the same effects. Methadone is also used in managing severe chronic pain, owing to its long duration of action, extremely powerful effects, and very low cost. Methadone was introduced into the United States in 1947 by Eli Lilly and Company.

Methadone is useful in the treatment of opioid dependence. It has cross-tolerance with other opioids including heroin and morphine, offering very similar effects and a long duration of effect. Oral doses of methadone can stabilise patients by mitigating opioid withdrawal syndrome. Higher doses of methadone can block the euphoric effects of heroin, morphine, and similar drugs. As a result, properly dosed methadone patients can reduce or stop altogether their use of these substances.

As with other opioid medications, tolerance and dependence usually develop with repeated doses. There is some clinical evidence that tolerance to analgesia is less with methadone compared to other opioids; this may be due to its activity at the NA recepetor. Tolerance to the different physiological effects of methadone varies; tolerance to both analgesic properties and euphoria develops quickly, whereas tolerance to constipation, sedation, and respiratory depression develops slowly (if ever).

Reference: http://en.wikipedia.org/wiki/Methadone