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important to understand that methadone does not actually “replace”
or “substitute” for other opioids. That’s why the terms replacement
and/or substitution therapy are inaccurate and misleading. Instead,
these medications are able to suspend withdrawal symptoms, decrease
drug craving behaviors and block the actions from other opioid drugs
such as heroin.

The pharmacological effects of methadone are markedly different
from those of heroin. Injected, snorted, or smoked, heroin causes an
almost immediate "rush" or brief period of euphoria that wears off
quickly, terminating in a "crash." The cycle of euphoria, crash, and
craving repeated several times a day leads to a cycle of addiction and
severe behavioral disruption.

These characteristics of heroin use result from the drug's rapid onset of
action and its short duration of action in the brain. An individual who
uses heroin multiple times per day subjects the brain and body to
marked, rapid fluctuations as the opiate effects come and go (figure 3).

The individual also will experience an intense craving for more heroin
to stop the cycle, fend off withdrawal and to reinstate the euphoria.
Ultimately however, when tolerance to the drug has been established,
the addicted person continues to use to avoid the pain of drug
withdrawal and to feel relatively normal.

Methadone has a very gradual and slow onset of action compared with
heroin. Because of this, patients stabilized on methadone do not
experience the euphoric “rush” (figure 4).

Methadone is metabolized more slowly than heroin and thereby allows
the brain and body to avoid the stressful ups and downs caused by
heroin. When on a stabilized dose during maintenance treatment,
there is also a marked reduction of the desire and craving for heroin.