Mechanism of action
Methadone is a synthetic opioid with a dual mechanism of action that distinguishes it from other opioids.
μ-Opioid Receptor Agonism
Methadone acts as a full agonist at μ-opioid receptors in the central nervous system, producing typical opioid effects including analgesia, euphoria, and respiratory depression. The μ-receptor activation inhibits adenylyl cyclase, decreases cAMP levels, and reduces neurotransmitter release, particularly of substance P and glutamate.
NMDA Receptor Antagonism
Unlike most opioids, methadone also acts as an antagonist at N-methyl-D-aspartate (NMDA) receptors. This property contributes to its efficacy in neuropathic pain and may reduce the development of opioid tolerance. The NMDA antagonism blocks excitatory glutamatergic neurotransmission and modulates central sensitization.
Cardiac Effects
Methadone blocks the hERG (human ether-à-go-go-related gene) potassium channel, which controls cardiac repolarization. This blockade prolongs the QT interval on ECG and increases the risk of torsades de pointes, a potentially fatal ventricular arrhythmia. This cardiac effect is dose-dependent and represents the most significant safety concern with methadone therapy.
- Methadone blocks hERG potassium channels causing QTc prolongation and torsades de pointes risk; requires ECG monitoring
- Long elimination half-life (24-36 hours) causes delayed toxicity; peak respiratory depression may occur days after dose escalation
- Analgesic duration (6-8 hours) is shorter than half-life requiring divided dosing for pain; once daily for maintenance therapy
- Metabolized by CYP3A4; inducers like rifampin precipitate withdrawal, inhibitors like azoles increase toxicity risk
- NMDA receptor antagonism in addition to μ-opioid agonism makes it effective for neuropathic pain and reduces tolerance
- Most methadone-related deaths occur in first week of therapy; start low and titrate slowly to avoid accumulation
- Standard urine opioid immunoassays do not detect methadone; requires specific methadone assay for monitoring compliance
- Neonatal opioid withdrawal syndrome is expected in infants born to mothers on methadone maintenance therapy
Indications
Methadone has two primary clinical applications approved by regulatory authorities.
Opioid Dependence Treatment
Methadone is a first-line agent for opioid maintenance therapy in opioid use disorder. Its long half-life (approximately 24-36 hours) allows once-daily dosing, preventing withdrawal symptoms and reducing cravings without producing significant euphoria at therapeutic doses. Methadone maintenance therapy has demonstrated efficacy in reducing illicit opioid use, criminal activity, HIV transmission, and overdose mortality. Treatment must be provided through certified opioid treatment programs under strict regulatory oversight.
Chronic Pain Management
Methadone is indicated for severe chronic pain that requires continuous opioid analgesia when alternative treatments are inadequate. It is particularly useful for neuropathic pain due to its NMDA receptor antagonism. The long duration of action provides sustained analgesia, though the analgesic effect typically lasts 6-8 hours initially, requiring multiple daily doses.
Off-Label Uses
Methadone has been used off-label for cancer pain, particularly when patients develop tolerance to other opioids or require opioid rotation. Its low cost and lack of active metabolites make it useful in renal impairment when other opioids are problematic.
Pharmacokinetics
Methadone exhibits complex and highly variable pharmacokinetics that contribute to its clinical challenges.
Absorption and Distribution
Methadone is well absorbed orally with bioavailability of approximately 70-90%. Peak plasma concentrations occur 2-4 hours after oral administration. The drug is highly lipophilic and extensively tissue-bound, with a large volume of distribution. Methadone is approximately 85-90% protein-bound, primarily to α1-acid glycoprotein.
Metabolism
Methadone undergoes hepatic metabolism primarily via CYP3A4, with contributions from CYP2B6, CYP2C19, and CYP2D6. It is metabolized to inactive metabolites (EDDP and EMDP) that are excreted in urine and feces. The extensive hepatic metabolism makes methadone highly susceptible to drug interactions with CYP inducers and inhibitors.
Elimination
Methadone has a highly variable half-life ranging from 8 to 59 hours (average 24-36 hours). This prolonged and unpredictable half-life creates a risk of drug accumulation with repeated dosing, particularly during the first week of therapy. Steady-state concentrations are typically reached after 5-7 days. The discrepancy between the duration of analgesic effect (6-8 hours) and elimination half-life (24-36 hours) requires careful dose titration to prevent accumulation and toxicity.
Special Populations
In hepatic impairment, methadone clearance may be reduced, requiring dose adjustment. Renal impairment does not significantly affect methadone clearance since the drug and its metabolites undergo both renal and fecal elimination. Methadone is minimally removed by dialysis due to its large volume of distribution and extensive protein binding.
Adverse effects
Methadone produces adverse effects typical of opioid agonists plus unique cardiac toxicity.
Cardiac Effects
QTc prolongation is the most serious adverse effect unique to methadone. The risk is dose-dependent and increases significantly at doses above 100-120 mg daily, though it can occur at any dose. QTc prolongation can lead to torsades de pointes, a polymorphic ventricular tachycardia that may degenerate into ventricular fibrillation and sudden cardiac death. Baseline and periodic ECG monitoring is recommended, particularly when initiating therapy or increasing doses.
Respiratory Depression
Respiratory depression is the most dangerous opioid effect and the primary cause of opioid overdose deaths. Risk is highest during initiation, dose escalation, and when combined with other CNS depressants. The long half-life of methadone means respiratory depression can be prolonged and may recur even after initial naloxone reversal.
Central Nervous System Effects
Common CNS effects include sedation, dizziness, confusion, and cognitive impairment. These effects are typically most pronounced during initiation and with dose increases. Chronic use may impair driving ability and increase fall risk, particularly in elderly patients.
Gastrointestinal Effects
Constipation occurs in the majority of patients and does not develop tolerance. Prophylactic stool softeners and stimulant laxatives are often necessary. Nausea and vomiting are common initially but usually improve with continued therapy.
Endocrine Effects
Chronic methadone use causes hypogonadism in both men and women through suppression of the hypothalamic-pituitary-gonadal axis. Men may experience decreased libido, erectile dysfunction, and decreased testosterone. Women may experience menstrual irregularities and decreased fertility. Routine monitoring of sex hormones may be appropriate in symptomatic patients.
Other Effects
Other adverse effects include diaphoresis (excessive sweating, often refractory to treatment), peripheral edema, urinary retention, pruritus, and xerostomia. Weight gain is common during maintenance therapy. Dental problems may occur due to xerostomia and, in the context of substance use disorder, possible neglect of oral hygiene.
Contraindications and warnings
Absolute Contraindications
Methadone is contraindicated in patients with known hypersensitivity to methadone or any formulation component, significant respiratory depression in unmonitored settings, acute or severe bronchial asthma in unmonitored settings, known or suspected gastrointestinal obstruction including paralytic ileus, and concurrent use of selegiline or within 14 days of MAO inhibitor use.
Cardiac Contraindications
Patients with congenital long QT syndrome should not receive methadone. Extreme caution is warranted in patients with structural heart disease, bradycardia, heart failure, or pre-existing QTc prolongation. Concurrent use with other QTc-prolonging medications significantly increases risk.
Black Box Warnings
Methadone carries multiple black box warnings. First, QTc prolongation and serious arrhythmias including torsades de pointes have been observed during treatment. Deaths have been reported. Second, respiratory depression is the chief hazard, particularly during initiation and dose escalation. Third, accidental ingestion, especially in children, can result in fatal overdose. Fourth, methadone has abuse potential and is a Schedule II controlled substance. Fifth, neonatal opioid withdrawal syndrome occurs with prolonged maternal use during pregnancy and requires management according to neonatology protocols.
Pregnancy and Lactation
Methadone is pregnancy category C. For pregnant women with opioid use disorder, methadone maintenance therapy is considered standard of care as the benefits of preventing illicit opioid use outweigh risks. However, neonatal abstinence syndrome (neonatal opioid withdrawal syndrome) is expected in neonates exposed to methadone in utero and requires specialized treatment. Methadone is excreted in breast milk but breastfeeding is generally encouraged in stable patients on maintenance therapy as benefits outweigh risks.
Special Populations
Elderly patients are at increased risk of respiratory depression and falls. Patients with hepatic impairment may require dose reduction due to decreased clearance. Use with extreme caution in patients with head injury, increased intracranial pressure, or altered mental status as methadone may obscure neurological assessment.
Drug interactions
Methadone is subject to numerous clinically significant drug interactions due to its metabolism and pharmacodynamic effects.
CYP3A4 Inducers
Strong CYP3A4 inducers (rifampin, phenytoin, carbamazepine, phenobarbital, St. John's wort) significantly increase methadone metabolism, reducing plasma concentrations and potentially precipitating opioid withdrawal. Patients may require dose increases of 30-50% or more. Close monitoring for withdrawal symptoms is essential when these agents are initiated.
CYP3A4 Inhibitors
CYP3A4 inhibitors (azole antifungals, macrolide antibiotics, protease inhibitors, grapefruit juice) decrease methadone metabolism, increasing plasma concentrations and toxicity risk. Dose reduction may be necessary. Particular caution is warranted with strong inhibitors like ketoconazole and ritonavir.
QTc-Prolonging Medications
Concurrent use of other QTc-prolonging drugs significantly increases arrhythmia risk. Important interacting medications include class IA and III antiarrhythmics (quinidine, procainamide, amiodarone, sotalol), certain antipsychotics (haloperidol, ziprasidone), fluoroquinolone antibiotics, azole antifungals, and tricyclic antidepressants. Avoid combinations when possible or increase ECG monitoring frequency.
CNS Depressants
Combination with benzodiazepines, alcohol, other opioids, sedative-hypnotics, or general anesthetics produces additive CNS and respiratory depression. This combination significantly increases overdose risk and mortality. The FDA issued a black box warning specifically addressing benzodiazepine-opioid combinations. Concurrent prescribing should be avoided unless no alternatives exist.
Serotonergic Drugs
Concurrent use with SSRIs, SNRIs, MAO inhibitors, tricyclic antidepressants, triptans, or linezolid may precipitate serotonin syndrome, a potentially life-threatening condition characterized by mental status changes, autonomic instability, and neuromuscular abnormalities.
Antiretroviral Agents
HIV protease inhibitors and non-nucleoside reverse transcriptase inhibitors interact significantly with methadone. Efavirenz, nevirapine, and some protease inhibitors reduce methadone levels and may precipitate withdrawal. Patients require close monitoring and potential dose adjustment when initiating or changing antiretroviral therapy.
Other Interactions
Urine acidification increases methadone renal clearance and may reduce efficacy. Naltrexone and other opioid antagonists precipitate acute withdrawal if administered to methadone-maintained patients. Mixed agonist-antagonist opioids (pentazocine, nalbuphine, butorphanol) may precipitate withdrawal and should be avoided.
Clinical pearls
- Methadone requires ECG monitoring. Obtain baseline ECG and check QTc before initiating therapy. Repeat ECG within 30 days of initiation, annually, and when dose exceeds 100 mg or following any dose increase. Consider more frequent monitoring if QTc >450 ms in men or >470 ms in women. Discontinue or reduce dose if QTc exceeds 500 ms.
- Analgesic duration differs from elimination half-life. Methadone's analgesic effect lasts 6-8 hours while the elimination half-life is 24-36 hours. For pain management, divide the total daily dose into multiple administrations. For opioid dependence, once-daily dosing is typical.
- Risk of delayed toxicity during initiation. Because of the long half-life and risk of accumulation, methadone toxicity (respiratory depression, excessive sedation) may not appear until 3-5 days after initiating therapy or increasing the dose. Start low and titrate slowly. Observe patients closely during the first two weeks.
- Methadone deaths are often related to inappropriate initiation. Most methadone-related deaths occur in the first few days of therapy when patients or prescribers are unfamiliar with its unique pharmacokinetics. Never exceed recommended initial dosing guidelines. For opioid-naive patients starting methadone for pain, typical initial doses are much lower than for opioid-tolerant patients.
- Urine drug screens may not detect methadone. Standard opioid immunoassays typically detect morphine metabolites and may not cross-react with methadone. A specific methadone assay is required to detect the drug. This is important when monitoring compliance in maintenance programs.
- Methadone clinics operate under special regulations. In most jurisdictions, methadone for opioid use disorder can only be dispensed through certified opioid treatment programs with strict regulations including observed dosing, particularly during early treatment. Prescribing for pain does not carry these restrictions but requires standard controlled substance protocols.
- Overdose may require prolonged naloxone treatment. Due to methadone's long half-life, opioid overdose reversal with naloxone may be temporary. Patients may require continuous naloxone infusion or repeated doses for 24-48 hours. Admission for observation is typically warranted.
- Methadone is effective for opioid rotation. When patients develop tolerance to other opioids, rotating to methadone can restore analgesia. Use published equianalgesic tables with caution as they may overestimate methadone potency. Start with conservative conversions (typically calculating the equianalgesic dose and then reducing by 75-90%) due to incomplete cross-tolerance.
- Electrolyte abnormalities increase arrhythmia risk. Hypokalemia and hypomagnesemia potentiate QTc prolongation. Correct electrolyte abnormalities before initiating methadone and monitor periodically, especially in patients with diarrhea, vomiting, or diuretic use.
QT-METHADONE for major risks: QTc prolongation, Torsades de pointes, Metabolism via CYP3A4, Elimination half-life is long (accumulation risk), Takes days to reach steady state, HERG channel blockade, Arrhythmia monitoring required, Duration of analgesia shorter than half-life, Overdose requires prolonged naloxone, Neonatal withdrawal, ECG required before and during therapy
Practice questions
1. A 34-year-old man with opioid use disorder is started on methadone maintenance therapy at a certified treatment clinic. He receives 40 mg daily for one week with good control of cravings. On day 9 of therapy, he is found unresponsive in his apartment. Emergency services note respiratory rate of 6 breaths/min and pinpoint pupils. Naloxone is administered with temporary improvement, but the patient becomes somnolent again 90 minutes later. Which pharmacokinetic property of methadone best explains this clinical course?
2. A 58-year-old woman with chronic neuropathic pain from diabetic polyneuropathy has inadequate relief with gabapentin and duloxetine. Her physician considers starting methadone. Before initiating therapy, an ECG is obtained showing QTc of 445 ms. Her current medications include metformin, atorvastatin, lisinopril, and fluoxetine. Which action is most appropriate before starting methadone?
3. A 42-year-old man on methadone maintenance therapy (80 mg daily) for 18 months is diagnosed with tuberculosis and started on rifampin-based therapy. Three days later, he presents to the clinic with lacrimation, rhinorrhea, piloerection, muscle aches, and diarrhea. Vital signs show blood pressure 158/95 mmHg, heart rate 108/min, and temperature 37.8°C (100°F). Pupils are dilated at 6 mm bilaterally. What is the most likely explanation for his presentation?
Frequently asked questions
How does methadone work?
Methadone works as a full μ-opioid receptor agonist and NMDA receptor antagonist. The opioid agonism produces analgesia and prevents withdrawal in opioid-dependent patients, while NMDA antagonism contributes to neuropathic pain relief and may reduce tolerance development. Its long half-life of 24-36 hours allows once-daily dosing for opioid maintenance therapy, preventing withdrawal symptoms without producing significant euphoria at therapeutic doses. The dual mechanism distinguishes it from most other opioids and contributes to its efficacy in chronic pain.
What are the side effects of methadone?
The most serious side effect is QTc prolongation leading to potentially fatal torsades de pointes arrhythmia, requiring ECG monitoring. Respiratory depression is the primary cause of overdose deaths, especially during the first week of therapy when drug accumulation occurs. Common side effects include constipation (occurs in most patients), sedation, dizziness, nausea, and excessive sweating. Chronic use causes hypogonadism with decreased libido and sexual dysfunction. Other effects include cognitive impairment, peripheral edema, dental problems from dry mouth, and weight gain during maintenance therapy.
Is methadone a controlled substance?
Yes, methadone is a Schedule II controlled substance in the United States due to high abuse potential. For opioid use disorder treatment, it can only be dispensed through certified opioid treatment programs under strict federal and state regulations, including observed dosing requirements. When prescribed for pain management, methadone follows standard Schedule II prescribing requirements but does not require specialized clinic settings. The medication requires secure storage, precise record-keeping, and is subject to diversion monitoring. Patients must provide identification and often undergo drug testing when receiving methadone in maintenance programs.
Methadone vs buprenorphine — what is the difference?
Methadone is a full μ-opioid agonist while buprenorphine is a partial agonist, making methadone more effective for severe opioid dependence but with higher overdose risk. Methadone can only be dispensed for addiction through specialized clinics requiring daily observed dosing initially, whereas buprenorphine can be prescribed in office-based settings with take-home doses. Methadone carries significant risk of QTc prolongation and cardiac arrhythmias that buprenorphine does not. Both medications are effective for opioid maintenance therapy, but methadone is preferred when higher opioid tolerance exists or when previous buprenorphine treatment failed.
Why does methadone prolong the QT interval?
Methadone blocks hERG (human ether-à-go-go-related gene) potassium channels in cardiac myocytes. These channels are responsible for the rapid delayed rectifier potassium current that controls cardiac repolarization during phase 3 of the action potential. When blocked, repolarization is delayed, prolonging the QT interval on ECG. This prolongation creates electrical instability that can trigger early afterdepolarizations and initiate torsades de pointes, a potentially fatal polymorphic ventricular tachycardia. The effect is dose-dependent and more pronounced when combined with other QTc-prolonging medications or in the presence of hypokalemia or hypomagnesemia.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: N07BC02
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.