Pharm 101: Oxycodone

Class

Opioid analgesic

Pharmacodynamics
  • Moderate opioid agonist that acts mainly on mu receptors in brain and spinal cord, but also outside of CNS
Pharmacokinetics
  • PO administration, IR/SR formulations
  • Good oral absorption
  • High Vd
  • Low first pass metabolism (compared with morphine)
  • Duration of action 3-4 hours, longer if controlled released formulation
  • Hepatic metabolism by P450 to oxymorphone and noroxycodone
  • Renal excretion of metabolites
Clinical uses
  • Treatment of moderate to severe pain
  • 10 mg morphine = 4.5 mg oxycodone
Adverse effects with acute use of opioid analgesics
  • Respiratory depression
    • Due to inhibition of brainstem respiratory controls allowing less response to hypercapnea
  • CNS:
    • Nausea/vomiting
    • Sedation
    • Dysphoria
    • Delirium
    • Seizures
  • CVS:
    • Bradycardia
    • Hypotension
  • GIT:
    • Urinary retention
    • Constipation
    • Biliary colic
  • Skin:
    • Urticaria
    • Pruritus
Adverse effects with chronic use of opioid analgesics
  • Endocrine:
    • Hypogonadism
    • Immunosuppression
    • Increased feeding
    • Increased GH secretion
  • CNS:
    • Withdrawal effects
    • Dependence*, tolerance**, abuse, addiction
    • Hyperalgesia

Strategies to reduce development of dependence include:

  • Smaller doses at longer intervals
  • Multi-modal analgesia with non-opioid analgesics
  • Use of modified/SR formulations
  • Establish goals at start of treatment
  • Frequent evaluation of ongoing requirements

Tolerance:

  • Does not occur with miosis, constipation and seizures
  • There is a moderate degree of tolerance to bradycardia
Precautions/contraindications of opioid analgesics
  • Use of pure agonists with weak partial agonists:
    • e.g. buprenorphine with morphine
    • Can reduce analgesia, and induce withdrawal
  • Head injury:
    • CO2 retention secondary to respiratory depression causes cerebral vasodilation
  • Pregnancy:
    • Chronic use may precipitate dependence in fetus
  • Impaired pulmonary function:
    • Risk of acute respiratory failure
  • Impaired hepatic or renal function
  • Endocrine disease:
    • Patients with Addison’s disease and hypothyroidism (myxedema) may have prolonged and exaggerated responses to opioids
  • Drug interactions:
    • Other sedative-hypnotics
    • Antipsychotic agents: increased sedation and accentuation of cardiovascular effects (antimuscarinic and alpha blocking actions)
    • MAOIs: high incidence of hyperpyrexic coma, hypertension
Further reading
References
Pharm 101 700

Pharmacology 101

Top 200 drugs

MBBS CCPU (RCE, Biliary, DVT, E-FAST, AAA) Rob is an Emergency Medicine Advanced Trainee based in Melbourne, Australia. He has special interests in medical education, ECG interpretation, and the use of diagnostic and procedural ultrasound in the undifferentiated and unwell patient.

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