Methadone can stay in the system for a long time. The elimination half-life of this particular medication is 8 to 59 hours. This means that the level of methadone in the blood is decreased by 50% every 8 to 59 hours. For any particular person, it is hard to tell exactly how long this will stay in the blood since this can accumulate in the liver as well as in other tissues.
The length of time methadone can be detected in your system varies from person to person. Contributing factors include strength and frequency of dose taken, height and weight, overall body metabolism, amount of exercise, and fluid intake.
It appears that methadone will leave your system at different times depending on how long you've been using it. A regular user can look forward to at least 7 to 10 days. A one time user can plan on 3 to 5 days. Sometimes it could take longer for either type of user.