Methadone is used as a substitute for heroin and morphine to treat opiate addiction. To understand what methadone does, it is first necessary to understand how opiates act on the body.
Heroin and morphine are opiates. They are both derived from opium, a product of the poppy plant. These drugs interact with the opiate receptors in the brain. The reaction of opiates in the brain causes sedation, analgesia (an inability to feel pain), and a euphoric (very happy), "high" sensation. It is because of these effects that opiates are considered addictive and are frequently abused.
Methadone is similar to morphine and opium in that it produces the same effects. The effects of methadone on the body last longer, however, than with opiates. It is the long-lasting effect of morphine that has made it a good treatment for opiate addiction.
In addition to having longer-lasting effects, methadone's withdrawal symptoms are much less severe than with opiates. Methadone also acts as a blocker in the brain so that addicts are less likely to go back to heroin because it will not give them the usual high.
Once a person is addicted to methadone, the standard treatment is to slowly wean them off the drug. This deliberate withdrawal is made easier because of methadone's less-severe withdrawal symptoms.