Opioid analgesic

  • Synthetic strong opioid agonist that acts on the mu receptor
  • Many different routes of administration: IV/IM/IN, SC, transdermal patch, epidural
    • Transdermal, mucosal and IM absorption are good
  • Highly lipid soluble
  • High first pass metabolism (low bioavailability)
  • Redistribution half-life 15 minutes, elimination half-life 3 hours
  • Duration of action 1-2 hours
  • Hepatic metabolism by CYP450 3A4 with no active metabolites
    • Therefore is opioid of choice in renal failure
Clinical uses
  • Severe pain
  • 0.1mg fentanyl = 10mg morphine (100 times more potent)
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


  • 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

Pharmacology 101

Top 200 drugs

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.