Methadone Flashcards
MOA?
long acting mu receptor agonist
typical dosing regimen and doses?
Daily unless BD for pain reasons
60 - 100mg/day
how long to reach steady state?
3 - 10 days
starting doses and increments?
10 - 30mg depending on tolerance levels
A second dose of up to 10mg may be given 4 - 6 hours later if in significant withdrawal
Increase no more than 10mg per week
what formulations are available?
syrup, tablet, IV
what is the half life?
20 - 36 hours
risks?
accumulation respiratory depression central sleep apnoea prolonging QT interval (avoid if underlying structural cardiac disease or on medications that waste potassium) severe constipation dental problems
Advantages over buprenoprhine?
Clients tend to cycle slower on/off methadone increasing the duration of MATOD and thus increasing the chances of stabilising the health and social situation
What and when would be the main variation in dosing?
If a patient has pain issues or is a rapid metaboliser they may require BD dosing to cover their pain or prevent withdrawal
Time to peak effect?
2.5 - 4 hours
What is the inter-individual variability in metabolism?
Up to 17 times between individuals
When to consider transfer to suboxone?
when dose increases or divided doses do not eliminate withdrawal symptoms prior to next dose (this may happen in about 1/3rd of clients on 60mg/day)
Pharmacokinetics?
hepatic metabolism by CYP 3A4, 2D6, 2B6
How long to reach maintenance dose?
2 - 6 weeks
*50mg - 65mg of methadone technically should cover the equivalent receptors to 1000mg per day of morphine
How to manage missed doses?
If misses 4 doses in a row reduce dose by half to prevent overdose
If 6 missed doses they require re-stabilisation
Maximum dose at end of first week?
50mg
At what dose should QTc be assessed even if no risk factors for prolongation?
≥100mg (particularly > 120mg/day)
how to transfer from Methadone to suboxone?
1) Methadone dose must be less than 40mg
2) the last dose must >24hrs ago
3) initiate suboxone when withdrawal symptoms are evident (COWS above 5) and start with 4mg suboxone and repeat dose if require 1 - 4 hrs later if ongoing symptoms
4) increase dose by 4 - 8mg per day as required (should stabilise between 2 -3 days)
Transferring from suboxone to methadone?
- if suboxone <8mg then start with methadone 20 - 30mg
- if suboxone dose is 12mg or more start with methadone 40 - 60mg
- however these assumptions are not accurate and what should guide dosage is the clinical symptoms
What rate is appropriate to wean off methadone?
5mg per fortnight
What agents can increase toxicity of Methadone?
CYP3A inhibitors such as azole’s, ciprofloxacin, erythromycin and grapefruit juice
