Methadone

Methadone is used for the treatment of opioid use disorder.[20] It may be used as maintenance therapy or in shorter periods 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, and did not affect criminal activity or risk of death.[21]

Treatment of opioid-dependent persons with methadone follows one of two routes: maintenance or withdrawal management.[22] 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 specialized clinic, where they are observed for around 15–20 minutes post-dosing, to reduce the risk of diversion of medication.[23]

The duration of methadone treatment programs ranges 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. While starting doses may be adjusted based on the amount of opioids reportedly used, most clinical guidelines suggest doses start low (e.g., at doses not exceeding 40 mg daily) and are incremented gradually.[10][24] It has been found that doses of 40 mg per day were sufficient to help control the withdrawal symptoms but not enough to curb the cravings for the drug. Doses of 80 to 100 mg per day have shown higher rates of success in patients and less illicit heroin use during the maintenance therapy.[8] However, higher doses do put a patient more at risk for overdose than a moderately low dose (e.g. 20 mg/day).[11]

Methadone maintenance has been shown to reduce the transmission of bloodborne viruses associated with opioid injection, such as hepatitis B and C, and/or HIV.[10] The principal goals of methadone maintenance are to relieve opioid cravings, suppress the abstinence syndrome, and block the euphoric effects associated with opioids.

Chronic methadone dosing will eventually lead to neuroadaptation, characterised by a syndrome of tolerance and withdrawal (dependence). However, when used correctly in treatment, maintenance therapy has been found to be medically safe, non-sedating, and can provide a slow recovery from opioid addiction.[10] Methadone has been widely used for pregnant women addicted to opioids.[10]

Methadone is used as an analgesic in chronic pain, often in rotation with other opioids.[25][26] Due to its activity at the NMDA receptor, it may be more effective against neuropathic pain; for the same reason, tolerance to the analgesic effects may be less than that of other opioids.[27][28]

Methadone withdrawal symptoms are reported as being significantly more protracted than withdrawal from opioids with shorter half-lives.

When used for opioid maintenance therapy, Methadone is generally administered as an oral liquid. Methadone has been implicated in contributing to significant tooth decay. Methadone causes dry mouth, reducing the protective role of saliva in preventing decay. Other putative mechanisms of methadone-related tooth decay include craving for carbohydrates related to opioids, poor dental care, and general decrease in personal hygiene. These factors, combined with sedation, have been linked to the causation of extensive dental damage.[36][37]

• Lightheadedness[38]
• Tearing of the eyes[38][39]
• Mydriasis (dilated pupils)[38]
• Photophobia (sensitivity to light)
• Hyperventilation syndrome (breathing that is too fast/deep)
• Runny nose[39]
• Yawning
• Sneezing[39]
• Nausea,[38][39] vomiting,[38][39] and diarrhea[38]
• Fever[39]
• Sweating[38]
• Chills[39]
• Tremors[38][39]
• Akathisia (restlessness)
• Tachycardia (fast heartbeat)[39]
• Aches[38] and pains, often in the joints or legs
• Elevated pain sensitivity
• Blood pressure that is too high (hypertension, may cause a stroke)

• Suicidal ideation
• Susceptibility to cravings[38]
• Depression[38]
• Spontaneous orgasm
• Prolonged insomnia
• Delirium
• Auditory hallucinations
• Visual hallucinations
• Increased perception of odors (olfaction), real or imagined
• Marked increase in sex drive
• Agitation
• Anxiety[38]
• Panic disorder
• Nervousness[38]
• Paranoia
• Delusions
• Apathy
• Anorexia (symptom)

Methadone has the following US FDA black box warning:[40]

• Risk of addiction and abuse
• Potentially fatal respiratory depression
• Lethal overdose in accidental ingestion
• QT prolongation[41]
• Neonatal opioid withdrawal syndrome in children of pregnant women
• CYP450 drug interactions
• Risks when used with benzodiazepines and other CNS depressants, including alcohol.
• A certified opioid treatment program is required under federal law (42 CFR 8.12) when dispensing methadone for the treatment of opioid addiction.

Most people who overdose on methadone show some of the following symptoms:

• Miosis (constricted pupils)[42]
• Vomiting[43]
• Spasms of the stomach and intestines[44]
• Hypoventilation (breathing that is too slow/shallow)[42]
• Drowsiness,[42] sleepiness, disorientation, sedation, unresponsiveness
• Skin that is cool, clammy (damp), and pale[42]
• Blue fingernails and lips[44]
• Limp muscles,[42] trouble staying awake, nausea
• Unconsciousness[42] and coma[42]

The respiratory depression of an overdose can be treated with naloxone.[39] Naloxone is preferred to the newer, longer-acting antagonist naltrexone. Despite methadone's much longer duration of action compared to either heroin and other shorter-acting agonists and the need for repeat doses of the antagonist naloxone, it is still used for overdose therapy. As naltrexone has a longer half-life, it is more difficult to titrate. If too large a dose of the opioid antagonist is given to a dependent person, it will result in withdrawal symptoms (possibly severe). When using naloxone, the naloxone will be quickly eliminated and the withdrawal will be short-lived. Doses of naltrexone take longer to be eliminated from the person's system. A common problem in treating methadone overdoses is that, given the short action of naloxone (versus the extremely longer-acting methadone), a dosage of naloxone given to a methadone-overdosed person will initially work to bring the person out of overdose, but once the naloxone wears off, if no further naloxone is administered, the person can go right back into overdose (based upon time and dosage of the methadone ingested).

As with other opioid medications, tolerance and dependence usually develop with repeated doses. There is some clinical evidence that tolerance to analgesia is less with methadone compared to other opioids; this may be due to its activity at the NMDA receptor. Tolerance to the different physiological effects of methadone varies; tolerance to analgesic properties may or may not develop quickly, but tolerance to euphoria usually develops rapidly, whereas tolerance to constipation, sedation, and respiratory depression develops slowly (if ever).[45]

Methadone treatment may impair driving ability.[46] Drug abusers had significantly more involvement in serious crashes than non-abusers in a study by the University of Queensland. In the study of a group of 220 drug abusers, most of them poly-drug abusers, 17 were involved in crashes killing people, compared with a control group of other people randomly selected having no involvement in fatal crashes.[47] However, there have been multiple studies verifying the ability of methadone maintenance patients to drive.[48] In the UK, persons who are prescribed oral methadone can continue to drive after they have satisfactorily completed an independent medical examination which will include a urine screen for drugs. The license will be issued for 12 months at a time and even then, only following a favourable assessment from their own doctor.[49] Individuals who are prescribed methadone for either IV or IM administration cannot drive in the UK, mainly due to the increased sedation effects that this route of use can cause.

In the United States, deaths linked to methadone more than quadrupled in the five-year period between 1999 and 2004. According to the U.S. National Center for Health Statistics,[50] as well as a 2006 series in the Charleston Gazette (West Virginia),[51] medical examiners listed methadone as contributing to 3,849 deaths in 2004. That number was up from 790 in 1999. Approximately 82 percent of those deaths were listed as accidental, and most deaths involved combinations of methadone with other drugs (especially benzodiazepines).

Although deaths from methadone are on the rise, methadone-associated deaths are not being caused primarily by methadone intended for methadone treatment programs, according to a panel of experts convened by the Substance Abuse and Mental Health Services Administration, which released a report titled "Methadone-Associated Mortality, Report of a National Assessment". The consensus report concludes that "although the data remains incomplete, National Assessment meeting participants concurred that methadone tablets or Diskets® distributed through channels other than opioid treatment programs most likely are the central factors in methadone-associated mortality."[52]

In 2006, the U.S. Food and Drug Administration issued a caution about methadone, titled "Methadone Use for Pain Control May Result in Death." The FDA also revised the drug's package insert. The change deleted previous information about the usual adult dosage. The Charleston Gazette reported, "The old language about the 'usual adult dose' was potentially deadly, according to pain specialists."[53]

Levomethadone (the L enantiomer) is a μ-opioid receptor agonist with higher intrinsic activity than morphine, but lower affinity.[56] Dextromethadone (the S enantiomer) has lower affinity to the μ-opioid receptor than Levomethadone. Both enantiomers bind to the glutamatergic NMDA (N-methyl-D-aspartate) receptor, acting as noncompetitive antagonists. Methadone has been shown to reduce neuropathic pain in rat models, primarily through NMDA receptor antagonism. NMDA antagonists such as dextromethorphan, ketamine, tiletamine and ibogaine are being studied for their role in decreasing the development of tolerance to opioids and as possible for eliminating addiction/tolerance/withdrawal, possibly by disrupting memory circuitry. Acting as an NMDA antagonist may be one mechanism by which methadone decreases craving for opioids and tolerance, and has been proposed as a possible mechanism for its distinguished efficacy regarding the treatment of neuropathic pain. Methadone also acted as a potent, noncompetitive α₃β₄ neuronal nicotinic acetylcholine receptor antagonist in rat receptors, expressed in human embryonic kidney cell lines.[57]

Methadone has a slow metabolism and very high fat solubility, making it longer lasting than morphine-based drugs. 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,[58][59] ranging from as few as 4 hours to as many as 130 hours,[60] or even 190 hours.[61] 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 withdrawal management 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.[60] 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.[62]

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).[4][63] 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.

• Methadone and its two main metabolites
• 
  Methadone
• 
  EDDP
• 
  EDMP

The most common route of administration at a methadone clinic is in a racemic oral solution, though in Germany, only the R enantiomer (the L optical isomer) has traditionally been used, as it is responsible for most of the desired opioid effects.[60] The single-isomer form is becoming less common due to the higher production costs.

Methadone is available in traditional pill, sublingual tablet, and two different formulations designed for the person to drink. Drinkable forms include ready-to-dispense liquid (sold in the United States as Methadose), and Diskets (known on the street as "wafers" or "biscuits") tablets which are dispersible in water for oral administration, used in a similar fashion to Alka-Seltzer. The liquid form is the most common as it allows for smaller dose changes. Methadone is almost as effective when administered orally as by injection. Oral medication is usually preferable because it offers safety, simplicity and represents a step away from injection-based drug abuse in those recovering from addiction. U.S. federal regulations require the oral form in addiction treatment programs.[64] Injecting methadone pills can cause collapsed veins, bruising, swelling, and possibly other harmful effects. Methadone pills often contain talc that, when injected, produces a swarm of tiny solid particles in the blood, causing numerous minor blood clots.[65][66] These particles cannot be filtered out before injection, and will accumulate in the body over time, especially in the lungs and eyes, producing various complications such as pulmonary hypertension, an irreversible and progressive disease.[67][68][69] The formulation sold under the brand name Methadose (flavored liquid suspension for oral dosing, commonly used for maintenance purposes) should not be injected either.[70]

Information leaflets included in packs of UK methadone tablets state that the tablets are for oral use only and that use by any other route can cause serious harm. In addition to this warning, additives have now been included in the tablet formulation to make the use of them by the IV route more difficult.[71]

Methadone and its major metabolite, 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP), are often measured in urine as part of a drug abuse testing program, in plasma or serum to confirm a diagnosis of poisoning in hospitalized victims, or in whole blood to assist in a forensic investigation of a traffic or other criminal violation or a case of sudden death. Methadone usage history is considered in interpreting the results as a chronic user can develop tolerance to doses that would incapacitate an opioid-naïve individual. Chronic users often have high methadone and EDDP baseline values.[72]

The protonated form of methadone takes on an extended conformation, while the free base is more compact. In particular, it was found that there is an interaction between the tertiary amine and the carbonyl carbon of the ketone function (R₃N ••• >C=O) that limits the molecule's conformation freedom, though the distance (291 pm by X-ray) is far too long to represent a true chemical bond. However, it does represent the initial trajectory of attack of an amine on a carbonyl group and was an important piece of experimental evidence for the proposal of the Bürgi–Dunitz angle for carbonyl addition reactions.[73]

Brand names include Dolophine, Symoron, Amidone, Methadose, Physeptone, Metadon, Metadol, Metadol-D, Heptanon and Heptadon among others.

In the US, generic methadone tablets are inexpensive, with retail prices ranging from $0.25 to $2.50 per defined daily dose.[86]

Methadone maintenance clinics in the US may be covered by private insurances, Medicaid, or Medicare.[87] Medicare covers methadone under the prescription drug benefit, Medicare Part D, when it is prescribed for pain, but not when it is used for opioid dependence treatment because it cannot be dispensed in a retail pharmacy for this purpose.[88] In California methadone maintenance treatment is covered under the medical benefit. Patients' eligibility for methadone maintenance treatment is most often contingent on them being enrolled in substance abuse counseling. People on methadone maintenance in the US either have to pay cash or if covered by insurance must complete a pre-determined number of hours per month in therapeutic groups or counseling.[89] The United States Department of Veteran's Affairs (VA) Alcohol and Drug Dependence Rehabilitation Program offers methadone services to eligible veterans enrolled in the VA health care system.[90]

Methadone maintenance treatment (MMT) cost analyses often compare the cost of clinic visits versus the overall societal costs of illicit opioid use.[91][92] A preliminary cost analysis conducted in 2016 by the US Department of Defense determined that methadone treatment, which includes psychosocial and support services, may cost an average of $126.00 per week or $6,552.00 per year.[93] The average cost for one full year of methadone maintenance treatment is approximately $4,700 per patient, whereas one full year of imprisonment costs approximately $24,000 per person.[94]

Methadone is a Schedule I controlled substance in Canada and Schedule II in the United States, with an ACSCN of 9250 and a 2014 annual aggregate manufacturing quota of 31,875 kilos for sale. Methadone intermediate is also controlled, under ACSCN 9226 also under Schedule II, with a quota of 38,875 kilos. In most countries of the world, methadone is similarly restricted. The salts of methadone in use are the hydrobromide (free base conversion ratio 0.793), hydrochloride (0.894), and HCl monohydrate (0.850).[95] Methadone is also regulated internationally as a Schedule I controlled substance under the United Nations Single Convention on Narcotic Drugs of 1961.[96][97]

In the United States, prescription of methadone requires intensive monitoring and must be obtained in-person from an Opioid Treatment Program—colloquially known as a 'methadone clinic'—when prescribed for opioid use disorder (OUD).[8] According to federal laws, methadone cannot be prescribed by a doctor and obtained from a pharmacy in order to treat addiction. Because of its long half-life, methadone is almost invariably prescribed to be taken in a single daily dose. At nearly all methadone clinics in the US, patients must visit a clinic to receive and take their dose under the supervision of a nurse. Both patients who are new to methadone treatment and high-risk patients—such as those who are using drugs and alcohol, including cannabis—must visit the clinic daily.[98][99]

In Russia, methadone treatment is illegal. In 2008, Chief Sanitary Inspector of Russia Gennadiy Onishchenko, claimed that Russian health officials are not convinced of the methadone's efficacy in treating heroin and/or opioid addicts. Instead of replacement therapy and gradual reduction of illicit drug abuse, Russian doctors encourage immediate cessation and withdrawal. Addicts are generally given sedatives and non-opioid analgesics in order to cope with withdrawal symptoms.[100] Brazilian footballer assistant Robson Oliveira was arrested in 2019 upon arriving in Russia with methadone tablets sold legally in other countries for what was considered drug trafficking under Russian law.[101]

As of 2015, China had the largest methadone maintenance treatment program with over 250,000 people in over 650 clinics in 27 provinces.[102]