Morphine vs Oxycodone
Clinical Comparison
Clinical Context
Morphine remains the WHO-recommended first-line strong opioid for cancer pain and is the benchmark against which all opioids are measured. Oxycodone is preferred in renal impairment because it lacks the active metabolite M6G that accumulates dangerously with morphine. Both require careful titration and laxative co-prescription.
Drug Profiles
Morphine
Strong opioid analgesic (phenanthrene)
Mechanism
Agonist at mu, kappa, and delta opioid receptors in the CNS and periphery, modulating pain perception and emotional response to pain
Indications
- Severe acute pain
- Chronic cancer pain
- Acute pulmonary oedema (cautious use)
- Post-operative analgesia
- Palliative care
Common Doses
Oral IR: 5-10 mg every 4 hours; Oral MR: 10-30 mg BD; IV: 1-10 mg titrated; SC: via syringe driver in palliative care
Route
Oral, IV, SC, IM, rectal
Onset & Duration
IV onset 5-10 min; oral onset 30-60 min; IR duration 4 hours; MR duration 12 hours
Oxycodone
Strong opioid analgesic (phenanthrene)
Mechanism
Agonist primarily at mu opioid receptors, with some kappa receptor activity; synthetic thebaine derivative
Indications
- Moderate to severe pain
- Cancer pain
- Post-operative pain
- Chronic non-cancer pain (when other opioids fail)
Common Doses
Oral IR: 5 mg every 4-6 hours; Oral MR: 10 mg BD; IV: half the oral dose
Route
Oral, IV, SC
Onset & Duration
Oral onset 15-30 min; IV onset 5 min; IR duration 4-6 hours; MR duration 12 hours
Key Differences
| Category | Morphine | Oxycodone |
|---|---|---|
| Active metabolites | M6G accumulates in renal impairment — toxicity risk | No clinically significant active metabolites |
| Renal impairment | Avoid or reduce dose — M6G accumulation | Preferred strong opioid in renal impairment |
| Histamine release | Significant — causes itching, nausea, hypotension | Minimal — better tolerated |
| Potency (oral) | Reference standard (1x) | 1.5x more potent than oral morphine |
| Cost | Very inexpensive | More expensive |
| WHO cancer pain ladder | First-line strong opioid (step 3) | Alternative or second-line strong opioid |
Active metabolites
M6G accumulates in renal impairment — toxicity risk
No clinically significant active metabolites
Renal impairment
Avoid or reduce dose — M6G accumulation
Preferred strong opioid in renal impairment
Histamine release
Significant — causes itching, nausea, hypotension
Minimal — better tolerated
Potency (oral)
Reference standard (1x)
1.5x more potent than oral morphine
Cost
Very inexpensive
More expensive
WHO cancer pain ladder
First-line strong opioid (step 3)
Alternative or second-line strong opioid
Key Advantages
Morphine
- Gold standard strong opioid — extensive experience
- Multiple formulations and routes
- First-line for cancer pain (WHO ladder step 3)
- Cheap and widely available
- Metabolite morphine-6-glucuronide contributes to analgesia
Oxycodone
- No active metabolites — safer in renal impairment
- Less histamine release — less nausea and itching
- More predictable pharmacokinetics
- 1.5x more potent than oral morphine (equianalgesic: oxycodone 5 mg = morphine 7.5-10 mg oral)
Key Cautions
Morphine
- Active metabolite M6G accumulates in renal impairment — risk of toxicity
- Histamine release — more nausea, itching, and hypotension
- Respiratory depression (dose-dependent)
- Constipation — always co-prescribe laxatives
- Dependence and tolerance with prolonged use
Oxycodone
- Higher abuse potential — Schedule 2 controlled drug
- Constipation (similar to morphine)
- Respiratory depression
- More expensive than morphine
- Dependence and tolerance
Clinical Verdict
Start with morphine as the first-line strong opioid (it is well-established, cheap, and available in many formulations). Switch to oxycodone if the patient has renal impairment, morphine-related nausea/pruritus, or intolerable histamine-mediated side effects.
Medical Disclaimer: This comparison is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare professional before making prescribing decisions. Verify all drug information with current clinical guidelines (BNF, NICE, SmPCs).
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