Methadone has a typical elimination half-life of 15 to 60 hours with a mean of around 22. However, metabolism rates vary greatly between individuals, up to a factor of 100, ranging from as few as 4 hours to as many as 130 hours, or even 190 hours. This variability is apparently due to genetic variability in the production of the associated cytochrome enzymes CYP3A4, CYP2B6 and CYP2D6. Many substances can also induce, inhibit or compete with these enzymes further affecting (sometimes dangerously) methadone half-life. A longer half-life frequently allows for administration only once a day in opioid detoxification and maintenance programs. People who metabolize methadone rapidly, on the other hand, may require twice daily dosing to obtain sufficient symptom alleviation while avoiding excessive peaks and troughs in their blood concentrations and associated effects. This can also allow lower total doses in some such people. The analgesic activity is shorter than the pharmacological half-life; dosing for pain control usually requires multiple doses per day normally dividing daily dosage for administration at 8 hour intervals.The main metabolic pathway involves N-demethylation by CYP3A4 in the liver and intestine to give 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP). This inactive product, as well as the inactive 2-ethyl-5-methyl-3,3- 
diphenyl-1-pyrroline (EMDP), produced by a second N-demethylation, are detectable in the urine of those taking methadone.
It is on the World Health Organizations List of Essential Medicines. Medical uses
Opioid addiction
Methadone is used for the treatment of opioid use disorder. It may be used as maintenance therapy or in shorter periods for detoxification to manage opioid withdrawal symptoms. Its use for the treatment of addiction is usually strictly regulated. In the US, outpatient treatment programs must be certified by the federal Substance Abuse and Mental Health Services Administration (SAMHSA) and registered by the Drug Enforcement Administration (DEA) in order to prescribe methadone for opioid addiction. A 2009 Cochrane review found methadone was effective in retaining people in treatment and in the reduction or cessation of heroin use as measured by self-report and urine/hair analysis but did not affect criminal activity or risk of death.Treatment of opioid-dependent persons with methadone follows one of two routes: maintenance or detoxification. Methadone maintenance therapy (MMT) usually takes place in outpatient settings. It is usually prescribed as a single daily dose medication for those who wish to abstain from illicit opioid use. Treatment models for MMT differ. It is not uncommon for treatment recipients to be administered methadone in a specialist clinic, where they are observed for around 15–20 minutes post-dosing, to reduce the risk of diversion of medication.The duration of methadone treatment programs range from a few months to years. Given opioid dependence is characteristically a chronic relapsing/remitting disorder, MMT may be lifelong. The length of time a person remains in treatment depends on a number of factors.