Not Medical Advice: Methadone has been known to show up in urine samples six to ten weeks after the last dose. In most people it is normally only detectable from 1 to 8 days.
Many factors contribute to how long methadone stays in the system, including: an individual's body weight, metabolism, history of use/abuse and many other factors. It has a slow metabolism and very high fat solubility, so it lasts longer than morphine-based drugs. The withdrawal period is also more prolonged than with other opiates, spanning anywhere from two weeks to several months.
A new study recently found that giving methadone treatment to drug addicts reduces the risk that they will get HIV.