Canterbury DHB

Context

Methadone Information Sheet

See also: Management of Persistent Pain - Methadone

Methadone 5 mg tablets, Methadone 2 mg/mL, 5 mg/mL or 10 mg/mL oral liquid and Methadone injection 10 mg/1 mL for subcutaneous (or IM) use

This guide is for methadone when it is being used as an analgesic in palliative care or pain patients, not when in use for opioid dependence. Methadone is a potent opioid analgesic that has complex pharmacology, making it difficult to use safely. Methadone should not be initiated other than by an expert in palliative care or pain medicine, usually in an inpatient setting.

Pharmacology

Methadone acts as an agonist on:

It has no active metabolites and is excreted mainly by the faecal route. Unlike morphine, the half-life of methadone exhibits marked inter- and intra-individual variability. The elderly are particularly susceptible.

 

Methadone

Morphine

Oral availability

85%

20-40%

Plasma half-life

13 - 80 hours

2-3 hours

Metabolism

CYP 3A4 (2D6, 1A2) to inactive metabolite

Glucuronidation to active metabolites

Elimination

Faecal and slight renal (depending on urine pH)

Metabolite mainly renal

Interactions

CYP enzyme inducers (e.g. phenytoin, dexamethasone) may decrease efficacy of methadone CYP enzyme inhibitors (e.g. fluconazole fluoxetine) which may increase efficacy and adverse effects. The efficacy and adverse effects of drugs metabolised by CYP2D6 (e.g. nortriptyline and haloperidol) may be increased by methadone.

Methadone will also interact with other CNS depressants such as alcohol and benzodiazepines.

Indications

Methadone is mainly used in the treatment of opioid addiction. However, it is increasingly being used internationally for the treatment of moderate to severe pain, and particularly in patients who are experiencing adverse effects of morphine or those with moderate to severe renal impairment.

Advantages of methadone

Disadvantages of methadone

Oral administration

Methadone is available in liquid and tablet form. Liquid formulations are 2 mg/mL, 5 mg/mL, and 10 mg/mL. Only 5 mg tablets are available. We generally recommend using the liquid, both to reduce tablet numbers and aid compliance, particularly in the terminal phase.

Oral to subcutaneous conversion

Although the oral bioavailability of methadone is 85%, in practice, when converting from oral to subcutaneous methadone, a range of between 50-75% of the 24 hour oral dose is given over 24 hours. Discuss with an expert if needed. Methadone cannot be mixed with any other medications so must be given in a separate syringe driver.

Subcutaneous administration

Due to its irritant properties and the need for a separate syringe driver, the subcutaneous route is only used if unavoidable. Methadone injection is available as 10 mg/mL in 1 mL ampoules. Dilute with sodium chloride 0.9% rather than water. A more dilute solution in a 20 mL or 30 mL syringe can be helpful. The subcutaneous site may need to be changed daily.

Dosing

More information and advice is available

Information on the Toombs/Ayonide method for converting to methadone from morphine (or morphine equivalent) can be located in the Palliative Care Handbook.

For more information and advice within working hours, contact either the Christchurch Hospital Palliative Care Service on (03) 364 1473, or the Nurse Maude Hospice on (03) 375 4274. After hours contact the Christchurch Hospital operator on (03) 364 0640 and ask for the palliative care clinician on call.

About this Canterbury DHB document (58017):

Document Owner:

Kate Grundy (see Who's Who)

Issue Date:

March 2016

Next Review:

March 2018

Keywords:

Note: Only the electronic version is controlled. Once printed, this is no longer a controlled document. Disclaimer

Topic Code: 58017