Will a patient allergic to morphine (opioid analgesic) be allergic to hydromorphone (opioid analgesic)?

Cross-Reactivity Between Morphine and Hydromorphone Allergies

Patients with true allergies to morphine are likely to also be allergic to hydromorphone, as both are structurally related opioids, and cross-reactivity is possible. However, patients with pseudo-allergic reactions to morphine (such as histamine-related symptoms) may tolerate hydromorphone well since it causes little to no histamine release. 1

Understanding Opioid Allergies

True Allergies vs. Adverse Reactions

• True allergic reactions to opioids are rare 2
• Many reported "allergies" to morphine are actually adverse effects or pseudo-allergic reactions related to histamine release
• Common histamine-related reactions include:
  • Urticaria (hives)
  • Pruritus (itching)
  • Facial flushing
  • Hypotension

Structural Considerations

• Hydromorphone is a semi-synthetic congener of morphine 3
• It is structurally similar to morphine but 5-10 times more potent 1, 3
• Due to structural similarity, true allergic reactions to morphine may cross-react with hydromorphone

Clinical Decision Making

When Hydromorphone May Be Safe

• For patients with Type 2 pseudo-allergic reactions to morphine (urticaria, pruritus, facial flushing) 1
• These reactions are typically caused by morphine's ability to trigger histamine release
• Hydromorphone causes significantly less histamine release than morphine
• The American College of Critical Care recommends hydromorphone as a safe alternative in these cases 1

When Hydromorphone Should Be Avoided

• In patients with true IgE-mediated allergic reactions to morphine
• Signs of true allergic reactions include:
  • Anaphylaxis
  • Angioedema
  • Bronchospasm
  • Severe hypotension unrelated to rate of administration

Management Recommendations

For Patients with Morphine Allergy

1. Determine the nature of the "allergy":

   • If symptoms were limited to itching, flushing, or hives without respiratory or cardiovascular compromise, hydromorphone may be safely used 1
   • If symptoms included anaphylaxis, angioedema, or severe bronchospasm, avoid all structurally related opioids

2. Consider alternative opioid classes:

   • For patients with true morphine allergies, consider synthetic opioids like fentanyl which have different chemical structures 3
   • Oxycodone may also be considered as an alternative 3

3. If hydromorphone is used in patients with previous morphine reactions:

   • Start with a lower dose than calculated (25-50% of equianalgesic dose)
   • Monitor closely for signs of allergic reaction
   • Have emergency medications (epinephrine, antihistamines, corticosteroids) readily available

Cautions and Considerations

• Both morphine and hydromorphone should be used with caution in patients with renal insufficiency due to potential accumulation of neurotoxic metabolites 3, 1
• The European Association for Palliative Care notes that up to 40% of patients may benefit from changing to an alternative opioid when experiencing troublesome adverse effects 1
• When switching between opioids, use conservative conversion ratios and titrate carefully to effect

Common Pitfalls to Avoid

• Misidentifying adverse effects as allergies: Many reported "allergies" are actually expected side effects like nausea, constipation, or sedation
• Assuming all opioids will cross-react: Different chemical classes of opioids may be tolerated despite allergy to another class
• Overlooking the potential for true cross-reactivity: Don't assume hydromorphone is always safe in patients with morphine allergy
• Failing to have emergency medications available: Always be prepared for potential allergic reactions when administering any opioid to a patient with previous opioid reactions