Module 2: Basic Pharmacology of Controlled Drugs and Substances
Pharmacokinetics of Opioids
The authors of this practice resource have made the decision not to provide specific conversion and dosing tables. The variability in published opioid equivalency and dosing tables is problematic for clinicians who are either prescribing initial opioid therapy or conducting an opioid rotation. Shaheen at al. (2009) conducted a review of existing clinical practice resources that included those available from pharmaceutical companies, standard prescriber reference documents and online conversion sources. Conversion ratios for oral to parenteral hydromorphone varied from 2:1 to 5:1 for example and conversion ratios from oral morphine to oral hydromorphone raged from 40-60: 6.5-7.5 to 10:2.5. The authors outline a thoughtful list of ‘problems and pitfalls’ in equianalgesic tables (p. 414):
- Failure to standardize a reference opioid (our note: usually morphine 10mg but no universally)
- Failure to address bidirectional differences (direction of conversion) in equianalgesia for certain opioids
- Inclusion of wide range of doses in the comparison
- Use of equianalgesic tables as references for other tables
- Equianalgesia between short and long acting opioids not at steady state
- Equianalgesia between opioid formulations and opioid/adjuvant combinations
- Use of equianalgesia determined by single-dose studies or acute pain
- Equianalgesic doses in organ failure
- Use of computations instead of clinical trial to determine equianalgesic ratios.
Authors of a similar review of 23 available smartphone apps had similar findings with the largest variability in conversion from oral codeine to morphine (Haffey, Brady & Maxwell, 2013). Only 48% of the apps provided references for conversions and 22% provided information regarding clinician involvement in app creation.
Clinicians must consider individual patient factors including comorbid conditions and organ function, age, medication interaction and metabolic/elimination status, apply a thorough knowledge of opioid pharmacology and indications before making a choice of equianalgesic and dosing tables to use in practice. Additionally, a critical examination of the practice resource must be an integral part of the clinician’s decision to apply existing tables in patient care (Anderson, Saiers, Abram & Schlicht, 2001; Periera, Lawlor, Vigano, Dorgan & Bruera, 2001).
- Anderson, R., Saiers, J. H., Abram, S., & Schlicht, C. (2001). Accuracy in equianalgesic dosing: conversion dilemmas. Journal of pain and symptom management, 21(5), 397-406.
- Haffey, F., Brady, R. R., & Maxwell, S. (2013). A comparison of the reliability of smartphone apps for opioid conversion. Drug safety, 36(2), 111-117.
- Pereira, J., Lawlor, P., Vigano, A., Dorgan, M., & Bruera, E. (2001). Equianalgesic dose ratios for opioids: a critical review and proposals for long-term dosing. Journal of pain and symptom management, 22(2), 672-687.
- Shaheen, P. E., Walsh, D., Lasheen, W., Davis, M. P., & Lagman, R. L. (2009). Opioid equianalgesic tables: are they all equally dangerous?. Journal of pain and symptom management, 38(3), 409-417.