Opioid Equivalence: Careful with Methadone

Pain management can be a tricky area. There are so many traps and pitfalls. Well, there is another major trap that I just recently became aware of. It turns out my favorite iPhone app (mediquations) can lead us all wrong, dangerously wrong.
Methadone is a long acting narcotic that can work very well for chronic pain. It can hit both the NMDA receptor and mu receptor, making it an excellent choice in neuropathic pain. Methadone is also unique due to its very long half life (up to 190 hours), yet it also has short acting effect and is dosed q6-8 hours for pain. Also, it absorbed in the stomach, unlike all of the other narcotics. Creating an interesting situation with patients with a gastric bypass. It explains why a patient with a gastric bypass overdosed on methadone when converted over to IV on our service. She was converted to IV correctly, but because of her gastric bypass, she was not really absorbing nearly as much as she was taking PO. So, when she was switched to IV, she suddenly taking a much higher dose of methadone. 
There is another major pitfall with methadone, non-linear potency. As the methadone dose increases, it becomes more potent! For example, if a patient is on oral morpine of 25 mg, mediquations will list the equivalent dose of methadone to be 16.67. At a dose of 50mg of oral morphine, it will give 33.33 (double of the prior), and at 200mg it will give 133.33 (8 times the dose). Mediquations assumes a linear potency, so as you double the morphine, the methadone will double. What is the correct answer? Not even close.
At 25 mg mediquations is about correct (should be about 12.5 is correct vs 16.67 by mediquations). At 50mg the true answer is actually also around 12.5! At 200mg, the closer answer is actually 25! This is an enormous difference! Mediquations provides the answer of 133.33 mg of methadone, when the true answer is actually closer to 25mg. Another difficulty with methadone is that the sedating effects are often delayed for 4-5 days, due to the long half life. So the patient will not be in trouble for several days, giving everyone false reassurance. As a general rule, wait for 4-5 days prior to titrating up the methadone dose. Since the titration process can be very long, be sure to use good breakthrough doses.

Below is a conversion table for methadone, taking into account its increasing potency with higher doses. 

24 hr Oral Morphine Equivalent----- Morphine:Methadone ratio
<30 mg --------------------------------- 2:1
31-99 mg ------------------------------- 4:1
100-299 mg ---------------------------- 8:1
300-499 mg ---------------------------- 12:1
500-999 mg ---------------------------- 15:1
1000-1200 mg ------------------------- 20:1 

The take home message for me is that methadone is a medication that requires experience, and is perhaps not something I will do without assistance. When I do use it I will certainly proceed with extreme caution! This is truly one of those medications that you start low and go slow!