In this episode, I’ll discuss the bioavailability of methadone.
Popular tertiary references suggest a conversion ratio of oral to IV methadone of 2:1, meaning 10 mg of oral methadone is equivalent to 5 mg of IV methadone.
But a team of researchers recently published a scoping review on the bioavailability of methadone in the American Journal of Health-System Pharmacy that draws a different conclusion.
The review included 50 patients from 6 different studies. Healthy patients as well as those with opioid use disorder, metastatic cancer, chronic pain from malignant or nonmalignant disease were represented in the review.
The researchers found the pooled mean bioavailability was 85.4% with a 95% confidence interval from 75.2% – 95.6%.
Furthermore, the review states:
In the 4 studies that provided individual patient–level data, F was >50% in 40 of 42 patient measurements (95.2%) and ≥75% in 33 of 42 patient measurements (78.6%).
The authors concluded that the evidence points to the bioavailability of methadone being more than 75%, and that an oral to IV conversion ratio of 1.3:1 is more appropriate than the commonly used 2:1 ratio. The authors also recommend that further research be done to identify a safe conversion ratio for oral to IV methadone.
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