A 55-year-old man on intravenous morphine for acute myocardial infarction develops intense pruritus over the face and upper chest within minutes of each dose. The nurse asks whether this is a true allergic reaction requiring discontinuation of morphine. Which statement best explains this finding?
- A It is an IgE-mediated type I hypersensitivity reaction and morphine should be stopped immediately
- B Morphine directly degranulates mast cells releasing histamine, a pharmacologic effect distinct from true allergy ✓
- C It is caused by morphine-induced leukotriene release and responds to montelukast
- D It results from accumulation of morphine-3-glucuronide causing direct neurogenic itching
Correct answer: B. Morphine directly degranulates mast cells releasing histamine, a pharmacologic effect distinct from true allergy
Explanation
Morphine directly triggers mast cell degranulation and histamine release by a non-immunologic mechanism. It is not IgE-mediated allergy. Antihistamines can help, and tolerance often develops. Option A is wrong because true anaphylaxis to morphine is rare. Option C incorrectly invokes leukotrienes. Option M3G accumulation causes neuroexcitation and allodynia, not pruritus.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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Written and medically reviewed by the StethoPrep medical team.