About This Calculator
Equianalgesic conversions used in this calculator are based on the American Pain Society guidelines and critical review papers regarding equianalgesic dosing.4,5,6,7 When possible, chronic-dosing studies have been used, including bidirectional and dose-dependent conversions.
There is an overall lack of data regarding most equianalgesic conversions, and there is a significant degree of interpatient variability. For this reason, reasonable clinical judgment, breakthrough (rescue) opioid regimens, and dose titration are of paramount importance.
reasonable clinical judgment, breakthrough (rescue) opioid regimens, and dose titration are of paramount importance
As a clinician, it is important to note that there are significant limitations to equianalgesic conversions and tables. While these equianalgesic tables are current the "best" solution, their limitations should be emphasized:
- Single-dose studies: Early studies determining equianalgesia were based on single doses, not chronic administation.8 Due to drug accumulation, half-life, tolerance, and active metabolites, subsequent chronic administration studies often vary greatly from the original single-dose data.
- Bidirectional conversions: When converting between certain opioids, the direction of conversion (eg, morphine to hydromorphone versus hydromorphone to morphine) will produce a different conversion ratio. These bidirectional differences are not captured in a traditional equianalgesic table.5,7
- Dose-dependent conversions: The conversion ratio of certain opioids can be dependent on the dose of the original opioid. In the case of converting morphine to methadone, methadone has a relative potency of 4:1 at lower morphine doses, but becomes much more potent (12:1) in patients converting from very high morphine doses.5,7
- Cross-tolerance: Many references recommend a cross-tolerance reduction between 25-50% when converting between unlike opioids.9 In patients with very high opioid requirements, the difference between 25% and 50% can be a very significant discrepancy.
- Equianalgesic Discrepancies: There are significant discrepancies in equianalgesic dosing tables, with even FDA-approved drug labels not demonstrating agreement.10 These discrepancies are a factor of both references using old data (single-dose studies) and an overall paucity of data in chronic dosing studies.
- Patient-specific factors: No equianalgesic table is able to take into account patient-specific factors -- primarily hepatic function, renal function, and age. Opioid metabolism and excretion do differ among the opioids; therefore, alterations in drug disposition will alter the relative potencies of different opioids.
When switching between opioids, equianalgesic conversions may overestimate the potency of the new opioid due to incomplete cross-tolerance. Incomplete cross-tolerance can occur due to variability in opioid binding. There is no evidence-based recommendation for an appropriate reduction. The American Pain Society guidelines and most pain experts recommend a dose reduction between 25-50% when converting between different opioids,9,11 with a consideration for little or no cross-tolerance reduction in patients with poorly controlled pain.5
Breakthrough (Rescue) Opioid Dosing
In patients receiving long-acting opioid formulations (SR, transdermal), a "rescue" dose for breakthrough pain is recommended.12 Generally, an immediate-release form of the same opioid is used (eg, morphine IR with morphine SR). The size of the breakthrough dose should be 5-15% of the patient's 24-hour baseline dose. In an inpatient setting, rescue doses can be provided IV every 15-30 minutes. Oral rescue doses can be offered as needed over the normal dosing interval of the drug (typically every 4 hours). As stated above, because equianalgesic tables are inherently inaccurate, the availability of breakthrough doses is paramount.
Opioid Dose Titration
Because equianalgesic tables are inherently inaccurate, dose titration to optimal effect is essential. Most guidelines recommend a dose increase or decrease of 25-50% from the previous day.12,13 In patients with long-acting formulations (SR), the amount of rescue doses needed in the previous day may be added to the patient's long-acting formulation. Because transdermal fentanyl has a delayed onset and onset of peak activity, consider titrating every 3 days.3
- Duragesic [package insert]. Titusville (NJ): Janssen Pharmaceuticals, Inc; 2012. DailyMed Monograph
- Skaer TL. Practice guidelines for transdermal opioids in malignant pain. Drugs. 2004;64(23):2629-38. PMID 15537367.
- Breitbart W, Chandler S, Eagel B, et al. An alternative algorithm for dosing transdermal fentanyl for cancer-related pain. Oncology (Williston Park). 2000;14(5):695-705. PMID 10853461.
- American Pain Society. Principles of analgesic use in the treatment of acute pain and cancer pain. 6th ed. Glenview, IL: American Pain Society; 2008.
- Anderson R, Saiers JH, Abram S, et al. Accuracy in equianalgesic dosing. conversion dilemmas. J Pain Symptom Manage. 2001;21(5):397-406. PMID 11369161.
- Pereira J, Lawlor P, Vigano A, et al. Equianalgesic dose ratios for opioids. a critical review and proposals for long-term dosing. J Pain Symptom Manage. 2001;22(2):672-87. PMID 11495714.
- Patanwala AE, Duby J, Waters D, et al. Opioid conversions in acute care. Ann Pharmacother. 2007;41(2):255-66. PMID 17299011.
- Knotkova H, Fine PG, Portenoy RK. Opioid rotation: the science and the limitations of the equianalgesic dose table. J Pain Symptom Manage. 2009;38(3):426-39. PMID 19735903.
- Chou R, Fanciullo GJ, Fine PG, et al. Clinical guidelines for the use of chronic opioid therapy in chronic noncancer pain. J Pain. 2009;10(2):113-30. PMID 19187889.
- Shaheen PE, Walsh D, Lasheen W, et al. Opioid equianalgesic tables: are they all equally dangerous? J Pain Symptom Manage. 2009;38(3):409-17. PMID 19735901.
- Levy MH. Pharmacologic treatment of cancer pain. N Engl J Med. 1996;335(15):1124-32. PMID 8813044.
- Cherny NI. Opioid analgesics: comparative features and prescribing guidelines. Drugs. 1996;51(5):713-37. PMID 8861543.
- Management of cancer pain: adults. Agency for Health Care Policy and Research. Am J Hosp Pharm. 1994;51(13):1643-56. PMID 7942889.