A Full Agonist with Long Half-Life
Methadone is a full agonist, which means it activates the opioid receptors completely. However, unlike heroin or fentanyl, methadone is absorbed slowly and has an exceptionally long half-life (24 to 36 hours). When administered in stable daily doses, it does not produce a "rush" or impair cognitive function, but instead creates a stable baseline that blocks withdrawal symptoms for a full day.
Regulating Cravings
By keeping the brain's opioid receptors consistently occupied, methadone prevents the intense cravings that often lead to relapse. It also blocks the effects of other opioids, meaning that if a patient on methadone attempts to use heroin, they will not experience a high.