Opioid · Analgesic
Morphine
Indications
- Acute severe pain needing an IV opioid, including PCA
- Analgesia in ventilated ICU patients (fentanyl or hydromorphone is often preferred)
- Intrathecal or epidural analgesia with preservative-free morphine, as after cesarean delivery
- ACS chest pain not relieved by nitrates
- End-of-life dyspnea (off-label)
Dosing
| Where | Dose |
|---|---|
| Other common pushes | Morphine: 2–4 mg IV every 5–15 min; histamine release, hypotension |
| Opioid equianalgesic doses | Morphine: 10 mg IV = 30 mg PO |
| Other ICU drugs | Morphine: Avoid: active metabolites accumulate (sedation, respiratory depression) |
On the quick cards
- Opioid analgesics and naloxoneNCLEX Drug Classes: Moderate to severe pain, overdose reversal
- Opioid equianalgesic dosesEquivalents and Conversions: Equal to morphine 10 mg IV
Mechanism of action
Morphine, the prototype opioid, is a full agonist relatively selective for the μ-opioid receptor, with weaker κ and δ activity. Through Gi/o proteins it lowers cAMP, opens GIRK potassium channels that hyperpolarize postsynaptic neurons and closes presynaptic calcium channels, so pain fibers release less transmitter. Being hydrophilic, it reaches brain and spinal receptors slowly, and its active metabolite M6G adds to and prolongs the effect.
Pharmacology
- CNS: analgesia, sedation, euphoria or dysphoria, miosis, nausea, cough suppression
- Respiratory: slower rate and blunted CO₂ response; histamine can provoke bronchospasm in asthma
- Cardiovascular: histamine release and vasodilation lower preload and BP, worse with fast pushes or hypovolemia
- GI: constipation, slowed gastric emptying (delays oral P2Y12 inhibitors, IMPRESSION), biliary spasm
- Other: itching and flushing, worst after neuraxial doses; urinary retention
Pharmacokinetics
| Onset | About 5 min IV; it crosses the BBB slowly |
|---|---|
| Peak | About 15–30 min IV |
| Duration | 3–5 h IV; up to 24 h after one neuraxial dose (Duramorph label) |
| Half-life | Usually 1.5–2 h (range 1.5–4.5 h, label) |
| Distribution | Vd 1.0–4.7 L/kg; little first-pass lung uptake; crosses the placenta |
| Protein binding | About 36% (label); other sources 20–35% |
| Metabolism | UGT2B7 glucuronidation, mainly hepatic, to M3G (about 90% of glucuronides; no analgesia, neuroexcitatory) and M6G (about 10%; an active μ agonist) |
| Elimination | Urine as glucuronides; 2–12% unchanged; about 10% in feces (label) |
| Organ failure | Kidney failure: M6G (sedation, respiratory depression) and M3G (myoclonus, seizures) accumulate; avoid or reduce |
| Bioavailability | Oral 20–40% (first pass), hence IV:PO about 1:3 |
Chemistry
- Natural phenanthrene alkaloid from the opium poppy: a rigid five-ring 4,5-epoxymorphinan whose 3-phenol and 6-alcohol carry the glucuronides
- Five chiral centers; only the natural (−) isomer is active
- pKa 7.9 (label), so about a quarter is un-ionized at pH 7.4; octanol:water partition only 1.42, so it enters the brain slowly and spreads upward in CSF after neuraxial doses
- Injection pH 2.5–6.5; neuraxial use needs preservative-free products (Duramorph, Infumorph); protect from light and discard if darker than pale yellow (label)
Adverse effects and cautions
In the one-pagers
- End of Life, Hospice, Pain and SleepOpioids: around the clock and breakthrough: Neurotoxicity: Jerking, hallucinations, pain from light touch or rising with each increase: hold escalation, report; kidney failure raises the risk with morphine
- Acute Abdomen and GI Surgical EmergenciesReading the acute abdomen: High-risk groups are the very old, the very young and anyone on corticosteroids or other immunosuppressants. These drugs blunt the inflammatory response, so a perforation can cause less pain, less tenderness and a smaller WBC rise than expected; a new tachycardia may be the main clue. Merck states that moderate IV analgesia (fentanyl 50-100 mcg or morphine 4-6 mg) does n
- Heart FailureDecompensated HF in the ICU: the first hours: Morphine is no longer routine for pulmonary edema: in the ADHERE registry (Peacock 2008) it was linked with more mechanical ventilation, ICU admission and in-hospital death, even after risk adjustment.
- Toxidromes and AntidotesOpioids and sympathomimetics: Opioids kill by hypoventilation, hypoxemia and arrest. Fentanyl is 50–100 times as potent as morphine (CDC), acts within minutes, can cause chest-wall rigidity with rapid IV doses and is missed by standard opiate screens; carfentanil and nitazenes may need repeated naloxone. Naloxone's half-life is 30–81 min (label), shorter than methadone or extended-release opioids,
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Sources
- FDA label: Morphine Sulfate Injection, Hikma (2026)
- FDA label: Duramorph (2025)
- PubChem: morphine
- Gabel 2022 (morphine glucuronides)
For education and exam preparation only. Follow your institution’s protocols. Typical adult values for study. Follow the order, the label and your institution's protocols.