Methadone is still the opioid with which I am most cautious. Yesterday in clinic we were talking about its use, conversion and risks as we were seeing a woman who was on Methadone 20 mg tid with a diagnosis of cervical cancer.
Methadone is the opioid most commonly associated with opioid deaths in the US from the CDC data. Lack of physician education about this drug and its pharmacology are one part of the problem, especially with its long and variable half life. There has also been considerable literature as to its conversion ratios from morphine and other opioids. How accurate is your conversion ratio?
A true story. Fourteen years ago, we converted a patient from 360 mg of oral morphine equivalents to methadone. All of the conversion charts used at that time suggested the conversion ratio of Morphine 30 mg = Methadone 20 mg. So dutifully the oncology fellow and the pharmacist spent some time doing the conversion, yes methadone 240 mg daily written as 80 mg three times per day. A few hours later the first dose, her respiratory rate dropped and she ended up ventilated in the ICU for two days….. Iatrogenic methadone overdose because that conversion ratio is based on a single dose conversion and does not take into factors of drug accumulation.
What dose should this woman have got? We went to the literature and the colleagues from Italy were doing the most work on this and I continue to use the Ripamonti conversion.
< 90 mg morphine equivalent 4:1
90-300 mg morphine equivalent 8:1
> 300 mg morphine equivalent 12:1
So this woman should have received, 30 mg/day or 10 mg tid. Our conversion charts have been corrected and significant education undertaken. But perhaps we need to do that again. Why? We went to a popular app that does opioid conversions (900,000 downloads as of last week). The suggested dose in senario? 360 mg morphine = 240 mg Methadone. WRONG!!!! We did try another app that came up with 30 mg!!
Some call me a geek. I love this technology stuff. But they do not replace the factual knowledge. Check the information and formulas that are being used. Notify the builders. Play your part in reducing the risk of opioids for your patients. Not the only cause but this could be a factor contributing to deaths associated with opioids.