Advanced Pharmacology · Topic 11 of 13

Opioid Prescribing & Conversion

Pharmacology · opioid analgesics

Opioid Prescribing & Conversion

Convert with the equianalgesic table, then cut 25–50% for cross-tolerance; choose kidney-safe opioids; and monitor sedation and breathing.

Key numbers

Morphine IV-to-oral ratio is 1:3IV:PO morphine 1:3

Reduce 25–50% when switching opioids↓ 25–50%

Reassess before exceeding 50 MME/day50 MME/day

01 · CalculateMorphine IV-to-oral ratio is 1:3

Equianalgesic doses: morphine 10 mg IV = morphine 30 mg oral = hydromorphone 1.5 mg IV = hydromorphone 7.5 mg oral = oxycodone 20 mg oral.

02 · The ruleReduce 25–50% when switching opioids

When switching opioids, calculate the equianalgesic dose of the new drug, then reduce it by 25–50% for incomplete cross-tolerance (methadone by 75–90%), and cover breakthrough pain with as-needed doses.

03 · AvoidKidney failure: avoid morphine, codeine, meperidine

In kidney failure avoid morphine and codeine (active metabolites accumulate, causing sedation, myoclonus and respiratory depression) and meperidine (normeperidine causes seizures); prefer fentanyl, or hydromorphone at a reduced dose.

Diagnosis & monitoring

Work-up

Monitor sedation and breathing on opioids must knowMonitor every inpatient on opioids for advancing sedation and respiratory depression; risk is highest with OSA, COPD, older age, renal or hepatic impairment, and benzodiazepines or gabapentinoids — use continuous electronic monitoring for high-risk patients.

Reassess before exceeding 50 MME/dayIn opioid-naive patients start at about 5–10 MME per dose (20–30 MME/day); increases beyond 50 MME/day bring progressively diminishing benefit relative to risk, so reassess carefully before going higher.

CDC morphine milligram equivalent factorsCDC MME factors: oxycodone 1.5, hydromorphone 5, hydrocodone 1, oxymorphone 3, codeine 0.15, tramadol 0.2, tapentadol 0.4; transdermal fentanyl mcg/h × 2.4. MME estimates risk — it is not a conversion tool.

Management

Orders

Titrate naloxone to breathing, not alertnessFor respiratory depression on chronic opioids, ventilate and titrate small IV naloxone doses (0.04–0.1 mg every 2–3 minutes, escalating as needed) to adequate breathing, not alertness, to limit withdrawal; redose, since it lasts 60–120 minutes.

Methadone: convert only with expertiseMethadone has a long, variable half-life and a nonlinear conversion ratio that rises with dose; toxicity can appear days after a change, so convert with expert help and reduce the calculated dose by 75–90%.

Fentanyl, methadone and buprenorphine accumulate leastIn severe renal insufficiency, accumulation is least likely with fentanyl and methadone; buprenorphine pharmacokinetics are unaffected by kidney failure or hemodialysis, making it another option.

Red flags & don'ts

Avoid harm

No fentanyl patch for opioid-naive patients avoidNever start extended-release opioids or a fentanyl patch for acute pain or in opioid-naive patients; reserve them for tolerance (≥ 60 mg oral morphine daily for ≥ 1 week). The patch peaks 18–24 hours after application.

Avoid opioids with benzodiazepines; offer naloxone interactionAvoid prescribing benzodiazepines and other sedatives with opioids when possible, account for sleep apnea and renal or hepatic insufficiency, and offer naloxone when overdose risk factors are present.

Start a bowel regimen with opioids complicationGive a daily stimulant (senna) and/or osmotic laxative (polyethylene glycol) with regular opioids — tolerance to constipation does not develop; use a peripheral mu-antagonist (methylnaltrexone, naloxegol) if refractory (see GI Motility Disorders).

The rules

Know these

No basal PCA infusion in opioid-naive adults must knowDo not add a routine basal infusion to IV PCA in opioid-naive adults: it does not improve analgesia and raises nausea and respiratory depression. PCA is safe partly because an oversedated patient cannot press the button.

Acute pain: immediate-release, lowest dose, few daysUse nonopioid therapy first for most acute pain; if an opioid is needed, give an immediate-release opioid at the lowest effective dose only while pain is severe — a few days or less often suffices (CDC 2022).

Morphine or hydromorphone?

Commonly confused

Equianalgesic IV dose	10 mg
Equianalgesic oral dose	30 mg
Kidney failure	Avoid — active glucuronides accumulate
Typical PCA demand dose	1 mg

More detail

Reference

Show 6 reference facts

Tramadol lowers the seizure threshold (higher risk in renal failure) and can cause serotonin syndrome with serotonergic drugs; in CKD lengthen the dosing interval and avoid it when GFR < 10 mL/min.

Codeine is a prodrug converted to morphine by CYP2D6: poor metabolizers get little analgesia, while ultrarapid metabolizers can develop morphine toxicity.

Typical PCA settings: demand dose morphine 1 mg or hydromorphone 0.2 mg, available as often as every 6 minutes, with a cumulative limit over a set interval (usually 4 hours).

A simplified oral conversion is 1 mg oral hydromorphone ≈ 4 mg oral morphine (table value 7.5 mg = 30 mg).

Long-half-life opioids (methadone, levorphanol) and modified-release forms take 4–5 half-lives or several days to reach steady state, so toxicity can be delayed after starting or increasing them.

Opioid-induced pruritus from neuraxial or parenteral opioids responds better to nalbuphine than to antihistamines, because much of it is a direct mu-receptor effect rather than histamine release.

Pearl

Calculate, then cut: cross-tolerance is incomplete, so reduce the new opioid's equianalgesic dose by 25–50%.

Sources · Merck Manual Professional — Table: Equianalgesic Doses of Opioid Analgesics · Merck Manual Professional — Treatment of Pain (opioid analgesics, PCA, adverse effects; updated Jun 2026) · Dowell D et al. CDC Clinical Practice Guideline for Prescribing Opioids for Pain — United States, 2022. MMWR Recomm Rep 2022;71(3):1–95 · Opioid Prescription in Patients With Chronic Kidney Disease: A Systematic Review. Cureus 2023;15:e45485 · ASPMN Guidelines on Monitoring for Opioid-Induced Advancing Sedation and Respiratory Depression: Revisions. Pain Manag Nurs 2020 (abstract via Europe PMC) · Merck Manual Professional — Opioid Toxicity and Withdrawal (naloxone 0.4 mg increments; 0.1 mg titration in long-term users) · Chou R et al. Management of Postoperative Pain: A Clinical Practice Guideline From the APS, ASRA and ASA. J Pain 2016;17:131–157 (recommends against routine basal infusion with IV PCA in opioid-naive adults) · Medical Dialogues — ASA/APS guidelines on the management of postoperative pain (summary of the 2016 recommendations; open copy read) · Kim HK, Nelson LS. Reducing the harm of opioid overdose with the safe use of naloxone: a pharmacologic review. Expert Opin Drug Saf 2015 (low-dose naloxone 0.04 mg, titrated, in opioid-dependent patients) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).