Initial assessment
When there is a need to initiate, adjust or rotate an opioid, the first step is to assess the clinical context of pain (acute vs chronic), previous opioid exposure and the patient’s profile. Age, degree of frailty, renal and hepatic function, presence of respiratory disease and drug interactions should be considered. The equianalgesic dose table is a decision-support tool, but it does not replace clinical judgement.
Concept of equianalgesic dose
An equianalgesic dose is the dose of an opioid that produces an analgesic effect similar to that of another opioid administered at a given dose. These equivalences are approximate and are usually based on oral morphine as the reference drug, allowing comparison between different opioids and routes of administration.
Incomplete cross-tolerance
When rotating between opioids, the presence of incomplete cross-tolerance should always be assumed. In practice, this means starting the new opioid with a reduction of the equianalgesic dose, usually between 25 and 50%, particularly in older, frail patients, those with relevant comorbidities or those receiving high doses. The dose should then be titrated according to clinical response.
Choice of opioid and route of administration
The choice of opioid and route should take into account pain intensity, ability to take oral medication, clinical stability and therapeutic goal. Intravenous and subcutaneous routes are useful in acute settings or when the oral route is not possible, whereas oral formulations are preferable in stable situations. Equivalences between routes should be used cautiously and always with an initial reduction when rotating.
Transdermal systems — specific considerations
Transdermal systems, such as fentanyl and buprenorphine, have a slow onset of action and are not indicated for acute or unstable pain. After patch initiation or dose adjustment, rescue analgesia may be required during the first 12–24 hours. Assessment should be performed after steady state is reached, avoiding premature dose adjustments.
Limitations of equivalences
Some opioids show greater interindividual variability. Codeine and tramadol depend on CYP2D6 metabolism, making analgesic effect less predictable. Tapentadol has an additional noradrenergic mechanism, so equivalences are more context-dependent. Methadone does not follow a linear dose-equivalence relationship and should not be converted using simple tables.
Titration and safety
After the initial conversion, gradual titration is essential, with reassessment of pain and adverse effects during the first 24–72 hours. Sedation, respiratory rate, cognitive status and constipation should be monitored. Rescue analgesia and preventive measures for adverse effects should be considered from the outset.
Final message
The equianalgesic dose table should be regarded as a clinical support tool. Safe opioid prescribing requires individualization, initial dose reduction during rotation, careful titration and continuous monitoring, ensuring an appropriate balance between pain control and patient safety.
