Opioid agonist therapy (OAT) is a treatment in which prescribed opioid agonists are given to patients who live with opioid use disorder (OUD). In the case of methadone maintenance treatment (MMT), methadone is used to treat dependence on heroin or other opioids, and is administered on an ongoing basis. The benefits of this treatment include a roughly 50% reduction in mortality among people with opioid use disorder, decreased substance use, improvements in social functioning, increased rates of retention in rehabilitative programs, more manageable withdrawal experience, cognitive improvement, and lower HIV transmission. The length of OAT varies from one individual to another based on their physiology, environmental surroundings, and quality of life.
Terminology Other terms that appear in the professional and popular literature include opioid replacement therapy, medication-assisted treatment (MAT), and medications for opioid use disorder (MOUD).
Biological understanding An opioid is considered a ligand, which is an ion or a molecule. An opioid ligand travels to the brain and attaches itself to an opioid receptor, which begins the effects of opioids. The mesolimbic system, which is the biological system that moderates the feeling of reward generated by dopamine, is the main system that is affected by opioids. Opioids stimulate the mesolimbic system to release a large amount of dopamine in the brain, which increases the effects of opioids: euphoria and numbness. The difference between an opioid and an opioid agonist is that opioids induce more intense effects and stay in the brain for a short amount of time. Conversely, an opioid agonist induces minimal effects and stays in the brain for a long time, which prevents the opioid user from feeling the effects of natural or synthetic opioids. However, the opioid receptors are still being used when an opioid agonist attaches, which prevents the effects of opioid withdrawal and can help prevent relapse. The two most common opioid agonists are methadone and buprenorphine.
Methadone Methadone is an opioid agonist that binds to the same receptors in the brain as heroin and other opioids. Introduced as an analgesic in the US in 1947, methadone has been used in maintenance treatment—also known as substitution treatment, or drug replacement therapy—since 1964. Methadone treatments usually last for multiple years, although they can last for decades. A dose of methadone often minimizes the effects of withdrawal for approximately 24 hours and the lowest optimal dose is 60 mg. Methadone functions via competitive antagonism; while the prescribed agonist is in the opioid user's body, the use of illicit opioids (illicit heroin or fentanyl) will not produce the effects of illicit opioids. Methadone has a slower onset than illicit opioids and it produces less effects than illicit opioids. Side effects of methadone may include "constipation, weight gain, reduced libido, and irregular menses".(p. 467) Methadone maintenance reduces the cravings for other opioids, and reduces the risk of fatal overdose from street drugs since the purity and strength of methadone is known, whereas substances obtained from the street vary significantly in strength and purity. Therapeutic dosing is contingent upon individual patient needs, with a dosage range generally between 20 and 200 mg. Doses are unsafe for opioid-naive individuals, and administration of methadone is gradually increased to reach a therapeutic dose under medical supervision to reduce the risk of overdose. The amount of oral methadone a patient will require is dependent on the amount and power of opioids they consumed prior to initiating treatment, with an assessment in the mid-2000s (prior to the widespread introduction of fentanyl into street heroin supplies in the US) finding that 1 gram of street heroin is roughly equivalent to 50 to 80 mg of methadone. Methadone is taken either orally as a mixture of 1 mg/1ml supplied as a red or clear liquid, or as a mixture containing 10 mg of methadone in 1ml of liquid (green color) or 20 mg in 1ml of liquid (brown color). The latter is often used when a person is on a large amount of methadone, and is rarely permitted for unsupervised consumption. Since the formulations are not as viscous as the 1 mg/1ml mixture, they are more prone to misuse since they are easier to inject, and due to the high risk of overdose if diverted to an individual not accustomed to such a large dose. Methadone also comes in 40 mg dispersible tablets called "diskettes", as well as 5 and 10 mg pills that are round or "coffin" shaped. These pills are only given in hospital settings. Methadone can also be delivered by either IV or IM injection, as well as ampoules which come in various strengths ranging from 10 mg up to 50 mg. This method is often used for individuals who have a "needle fixation" and who would otherwise revert to using IV heroin. With the emergence of other medications for the treatment of opioid addiction such as buprenorphine and long-acting naltrexone, MMT is no longer the dominant medically assisted addiction treatment. Methadone maintenance has been termed "a first step toward social rehabilitation" because it increases the retention of patients in treatment, relieves them from the need to find, buy, and use multiple daily doses of street opioids, and offers a legal medical alternative. Methadone is one of the most researched treatments for opioid use disorder and has significant research support for its efficacy.
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