Opioids - Equianalgesic Dosages - GlobalRPH (2024)

Opioids - Equianalgesic Dosages

Published equianalgesic ratios are considered crude estimates at best and therefore it is imperative that careful consideration is given to individualizing the dose of the selected opioid. Dosage titration of the new opioid should be completed slowly and with frequent monitoring.

Factors that must be addressed during the conversion process include: Age of the patient or presence of coexisting conditions. Use additional caution with elderly patients (65 years and older), and in patients with liver, renal, or pulmonary disease.

Conversion ratios in many equianalgesic dosing tables do not apply to repeated doses of opioids.

Review the concept of incomplete cross-tolerance:

D. McAuley: "Incomplete cross-tolerance relates to tolerance to a currently administered opiate that does not extend completely to other opioids. This will tend to lower the required dose of the second opioid. This incomplete cross-tolerance exists between all of the opioids and the estimated difference between any two opiates could vary widely. This points out the inherent dangers of using an equianalgesic table and the importance of viewing the tabulated data as approximations. Many experts recommend - depending on age and prior side effects - reducing the dose of the new opiate by 33 to 50 percent to account for this incomplete cross-tolerance. (Example: a patient is receiving 200mg of oral morphine daily (chronic dosing), however, because of side effects a switch is made to oral hydromorphone 25 - 35mg daily - (this represents a 33 to 50 percent reduction in dose compared to the calculated 50mg conversion dose produced via the equianalgesic calculator). This new regimen can then be re-titrated to patient response. In all cases, repeated comprehensive assessments of pain are necessary in order to successfully control the pain while minimizing side-effects."

The amount of residual drug in the patient's system must be accounted for. Example: fentanyl will continue to be released from the skin 12 to 36 hours after removal of the patch. Residual effects from discontinued long-acting formulations should also be assessed before converting a patient to a new opioid.

The use of high but ineffective doses of a previous opioid may result in overestimation of the converted opioid.

Ideally, methadone conversions (especially patients who were previously receiving high doses of an opioid) should only be attempted in cooperation with a pain specialist or a specialist in palliative medicine.

Meperidine should be used for acute dosing only and not used for chronic pain management (meperidine has a short half-life and a toxic metabolite: normeperidine). Its use should also be avoided in patients with renal insufficiency, CHF, hepatic insufficiency, and the elderly because of the potential for toxicity due to accumulation of the metabolite normeperidine. Seizures, confusion, tremors, or mood alterations may be seen. In patients with normal renal function, total daily doses should not exceed 600mg/24hrs.

Methadone Chronic dosing:
0-99 mg: 4:1
100-299 mg:8:1
300-499 mg: 12:1
500-999 mg: 15:1
>1000 mg: 20:1

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Accessed: October 2010.

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