Methadone

Introduction

Methadone is an opioid analgesic. Methadone hydrochloride is used in the treatment of moderate to severe pain; it may be of use for those patients who have excitation or exacerbation of pain with morphine. Methadone is also used in the management of opioid dependence.

Generic Name 藥名	HA Code 藥物代碼	Classification藥物分類
Methadone tablet 5mg		DD

Mechanism of Action

Equivalent doses of opioid analgesics

Analgesic/Route	Dose
Codeine: PO	100 mg
Diamorphine: IM, IV, SC	3 mg
Dihydrocodeine: PO	100 mg
Morphine: PO	10 mg
Morphine: IM, IV, SC	5 mg
Oxycodone: PO	6.6 mg
Tramadol: PO	100 mg

Route of Administration	PO (tablet form)
	IV

Dosage

For pain relief

Oral	starting dose range from 2.5 to 10mg every 6 to 8 hours or longer, adjusted as necessary.

Intramuscular, intravenous and subcutaneous route being recommended for prolonged use. The analgesic effect of methadone begins about 10 to 20 minutes after parenteral injection and about 30 to 60 minutes after oral doses, the effect of a single dose usually lasting about 4 hours. As accumulation occurs with repeated doses, the effects more prolonged. Consequently, to avoid the risk of opioid overdose, it is recommended that in prolonged use methadone should not be given more than twice daily.

For the treatment of opioid dependence,

Oral	starting dose of 10 to 40 mg daily, to suppress signs of opioid withdrawal. Most patients in such maintenance programmes are stabilized on once-daily doses of 60 to 120 mg.

Methadone is not licensed for use in children.

Severe pain	5-10mg q6-8h PRN
Opioid dependence	initally 10-30mg on first day, adjusted according to response

Side Effects

Methadone has a more prolonged effect than morphine and readily accumulates with repeated doses. Repeated doses of methadone may result in marked sedation.

Euphoria, nausea and vomiting, respiratory depression, constipation and cough suppression, QT prolongation, increase intracranial pressure.

Pharmacokinetics

Oral bioavailability	Methadone is readily absorbed from the gastrointestinal tract and after subcutaneous or intramuscular injections. It crosses the placenta and is distributed into breast milk.
Onset of action	In addition to marked interindividual variations there are differences in the pharmacokinetics of methadone after single or multiple doses. Careful adjustment of dosage is necessary with repeated doses.
Metabolism	Methadone is metabolized in the liver, and is excreted in the urine.
Elimination half-life	It has a prolonged half-life and is subject to accumulation. Elimination half-lives vary considerably ( a range of 15 to 60 hours has been quoted).

Drug Management

Efficiency

Pain control

Safety

mental status, blood pressure, respiratory drive, and misuse / overuse.

Precautions

methadone should be given with caution to patients at risk of developing prolongation of the QT interval including those with cardiac or hepatic disease, with hypokalaemia or other electrolyte imbalance. ECG monitoring is recommended before starting methadone treatment in these patients.

Drug Interaction

Increase risk of serotonin syndrome when used with SNRI, TCA, linezolid.

Caution

History of cardia conduction abnormalities Tolerance and dependence

Contraindication

Phaeochromocytoma

FAQ

How should I take the tablet?

Follow the doctor's prescription, Take with or without food Take the medication with plenty of water Can be crushed

What should I avoid while taking?

Avoid abrupt withdrawal

What happen if I miss a dose?

Take it as soon as you remember. If it is near the time of the next dose, skip the missed dose. Take your next dose at the regular time. Do not double the dose to catch up.