Following intravenous morphine, a patient develops flushing, itching over the face and trunk, and a fall in blood pressure. Which feature distinguishes this reaction from true opioid allergy?

• A It indicates conversion to opioid dependence
• B It is IgE mediated and mandates lifelong avoidance of all opioids
• C It occurs only with synthetic opioids such as fentanyl
• D It results from direct mast cell degranulation and can be prevented by prior antihistamine ✓

Explanation

Morphine directly triggers mast cell histamine release through a non-immunologic mechanism, producing pruritus, flushing, and hypotension. This is a pseudoallergic reaction, common with morphine and pethidine, and rare with fentanyl and other highly lipophilic phenylpiperidines. It is not IgE mediated, so switching to fentanyl rather than avoiding all opioids is the practical response.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.