Methadone dosage

There are a number of factors that affect methadone dosage for a patient and before prescribing a dose it is important to be aware off them. While there are a number of ways of giving methadone to a patient, most of it is given orally in tablet form or a solution. This oral methadone is quickly absorbed in the body and has a long acting period.

When compared to morphine its bioavailability is more than 3 times and it half life is up to
10 times greater. Methadone is also very lipophilic and gets distributed to major organs quickly resulting in substantial plasma concentrations from these organs and peak concentrations occur between two & an half to four hours after a patient taking it.

It is also important to note that while methadone may have a half-life of up to 30 hours its analgesic effects are only about 4 to 6 hours. This difference may have probable life threatening consequences for patients who use it like other opioids like morphine or oxycodone.

The dosing
When determining the methadone dosage you may approach it in two ways:
I) Opioid native patients
For opioid native patients a slow start approach is best where you begin slow and also go slow. It would therefore be recommended to start on about 2.5mg po every 8hrs which is a conservative and safe dose. Though in elderly patients just one dose of 2.5mg a day would be enough.
Any increase in dosage should be based on the patient’s response although an increase of  about 2.5mg per dose every 5 to 7 days advisable.

II) Opioid tolerant patients
Equianalgesic tables will show that about 15mg of morphine administered orally will be equivalent to 10mg of an oral methadone dose. However it is not enough to rely only on single dose tablets to transition a patient from other opioids to methadone as with repeated dosing methadone tends to have stronger analgesic effects which may lead to overmedication or even possibly an overdose which may not be detected immediately.

There are a number of equianalgesic dose ratios(EDR) that have developed for conversion to help with correlating the total daily opioid use before switching to methadone

EDR example 1:
(from Ripamonti et al..1998)
Morphine dose(mg/d)        30-90        90-100        300+
Morphine:Methadone EDR      4 :1          6 :1          8 :1

EDR example 2:
(from Mercadante et al..2001)
Morphine dose(mg/d)        30-90        90-100        300+
Morphine:Methadone EDR      4 :1          8 :1          12 :1

EDR example 3:
(from Ayonrinde, 2000)
Morphine dose(mg/d)        <100    101-300   300-600   601-800   801-1000   >1001
Morphine:Methadone EDR       3 :1       5 :1            10 :1    12:1         15:1          20:1

As can be seen from the 3 different methadone EDRs there are a number of inconsistencies. Therefore a methadone conversion nomogram was created:

The nomogram is used by looking at the current oral morphine dose on the x axis then moving up the curve and finding the matching methadone dose on the y axix

Special case dosing
There are a number of medical conditions in patients that would require special consideration when prescribing methadone. These include:

Patients with HIV/AIDS will usually be on a number of anti-retroviral drugs which may have unpredictable results on a patient’s methadone serum level. Therefore care should be taken in such cases and it is advisable that the patient’s HIV/AIDS care provider be consulted first.

Cardiac conditions
The medication taken by patients with cardiac conditions may interact with methadone and will require consideration for the particular type of medication the patient is on.

Patients with renal or hepatic failure
Patients with renal or hepatic failure will require special dose consideration due to the way the Methadone is cleared in the kidneys.

The elderly
Patients who are elderly may experience an exaggerated response to methadone and it is therefore advisable to start them on a low dose. They may also be taking medication that may interact with the methadone.