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Drug Hypersensitivity Reaction

Overview

A drug hypersensitivity reaction (DHR) is an abnormal immune‑mediated response to a medication that occurs at doses normally used for therapy. Unlike ordinary side effects, which are predictable and dose‑related, hypersensitivity reactions are unpredictable, can involve any organ system, and may be life‑threatening.

These reactions can affect anyone who takes a drug, but certain groups are more susceptible:

• Women (about 60–70 % of reported cases) <sup>1</sup>
• Patients with a personal or family history of allergies, asthma, or atopic dermatitis
• Individuals with certain HLA genotypes (e.g., HLA‑B*57:01 and abacavir hypersensitivity) <sup>2</sup>

Overall prevalence varies by drug class. For example, antibiotics cause DHR in ~10 % of users, while non‑steroidal anti‑inflammatory drugs (NSAIDs) account for ~2‑5 % of all adverse drug events (ADEs) <sup>3</sup>. In the United States, drug hypersensitivity contributes to an estimated 5–15 % of hospital admissions for drug‑related problems <sup>4</sup>.

Symptoms

Because DHR can involve any organ system, the clinical picture is broad. Symptoms usually appear within minutes to several weeks after drug exposure, depending on the underlying immune mechanism.

Cutaneous (skin) manifestations

• Urticaria (hives): pink, raised, itchy wheals that may migrate.
• Angio‑edema: swelling of the lips, eyelids, or airway; often painless but can obstruct breathing.
• Maculopapular rash: flat or slightly raised red lesions, often starting on the trunk.
• Stevens‑Johnson syndrome (SJS) / Toxic epidermal necrolysis (TEN): painful, blistering skin loss covering >10 % of body surface (TEN) or <10 % (SJS) and mucosal involvement.
• Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS): widespread rash with fever, facial swelling, and internal organ involvement.

Respiratory symptoms

• Wheezing, shortness of breath, or cough.
• Bronchospasm (as seen in anaphylaxis).

Cardiovascular signs

• Hypotension (low blood pressure).
• Arrhythmias or tachycardia.
• Cardiogenic shock (rare, seen in severe anaphylaxis).

Gastrointestinal complaints

• Nausea, vomiting, abdominal pain.
• Diarrhea.

Systemic / Other manifestations

• Fever or chills.
• Joint pain (arthralgia) or muscle aches.
• Renal involvement: acute interstitial nephritis.
• Hepatic involvement: drug‑induced hepatitis.
• Neurologic: headaches, dizziness, or, rarely, seizures.

Causes and Risk Factors

Immunologic mechanisms

Drug hypersensitivity falls into two broad categories:

1. IgE‑mediated (Type I) reactions: Immediate (within minutes to 2 hours) and can progress to anaphylaxis. Classic examples include penicillin, sulfonamides, and neuromuscular blocking agents.
2. Non‑IgE (T‑cell mediated) reactions: Delayed (hours to weeks) and include maculopapular eruptions, SJS/TEN, DRESS, and drug‑induced organ injury. These involve drug‑specific T‑cell activation (Type IV hypersensitivity).

Common drug classes implicated

• Antibiotics (β‑lactams, sulfonamides, fluoroquinolones)
• NSAIDs and aspirin
• Anticonvulsants (carbamazepine, lamotrigine)
• Allopurinol
• Antiretrovirals (abacavir, nevirapine)
• Chemotherapy agents (e.g., carboplatin)

Risk factors

• Previous drug allergy or multiple drug intolerances.
• Underlying atopic disease (asthma, allergic rhinitis, eczema).
• Viral infections at the time of drug exposure (e.g., HIV, hepatitis C), which can amplify immune response.
• Genetic predisposition (specific HLA alleles). For instance, HLA‑B*15:02 increases risk for carbamazepine‑induced SJS in Asian populations <sup>5</sup>.
• Dose and route of administration – intravenous exposure often leads to more severe immediate reactions.
• Concurrent medications that may act as cofactors (e.g., diuretics worsening NSAID reactions).

Diagnosis

Clinical assessment

Diagnosis begins with a detailed history:

1. Exact drug(s) taken, dose, route, and timing of onset.
2. Description of symptoms and their evolution.
3. Any previous drug reactions or known allergies.
4. Concomitant illnesses or infections.

Laboratory and skin testing

• Serum tryptase: Elevated 1–2 hours after anaphylaxis indicates mast‑cell activation.
• Specific IgE testing (e.g., ImmunoCAP): Useful for β‑lactam antibiotics and some NSAIDs.
• Skin prick or intradermal testing: Performed by allergy specialists for IgE‑mediated reactions. Positive results must be interpreted with caution because false positives are common.
• Patch testing: For delayed, T‑cell mediated reactions such as maculopapular rash, DRESS, or SJS/TEN. Typically applied 48 hours after the acute episode.
• Lymphocyte transformation test (LTT): Measures drug‑specific T‑cell proliferation; available in specialized centers.

Other investigations

• Complete blood count (CBC) – eosinophilia may suggest DRESS.
• Liver and renal function panels – assess organ involvement.
• Skin biopsy – can distinguish SJS/TEN from other rashes.
• Genetic screening – e.g., HLA‑B*57:01 testing before starting abacavir (CDC recommendation) <sup>6</sup>.

Diagnostic criteria

Validated algorithms such as the Naranjo Adverse Drug Reaction Probability Scale and the RegiSCAR scoring system for SJS/TEN help differentiate true hypersensitivity from coincidental illness.

Treatment Options

Acute management

• Immediate drug discontinuation: Stop the offending agent as soon as a DHR is suspected.
• Airway, breathing, circulation (ABCs): For anaphylaxis, secure the airway and administer high‑flow oxygen.
• Epinephrine: First‑line for anaphylaxis – 0.3 mg intramuscular (IM) into the mid‑outer thigh; repeat every 5–15 minutes if needed.
• Antihistamines: H1 blockers (diphenhydramine, cetirizine) for urticaria or angio‑edema.
• Corticosteroids: Oral or IV (prednisone 0.5 mg/kg or methylprednisolone) to reduce late‑phase reactions; evidence for preventing biphasic anaphylaxis is modest but commonly used.
• Bronchodilators: Albuterol inhalation for bronchospasm.
• Supportive care: IV fluids for hypotension, topical emollients for skin lesions, and wound care for SJS/TEN (often in burn units).

Specific therapies for severe delayed reactions

• SJS/TEN: Transfer to an intensive care or specialized burn unit; consider systemic cyclosporine (3 mg/kg/day) or intravenous immunoglobulin (IVIG 2 g/kg) – data are mixed but may improve survival.
• DRESS: Systemic corticosteroids (1 mg/kg prednisone) tapered over 6–8 weeks; monitor liver and kidney function.

Long‑term management

• Drug desensitization: In select cases (e.g., penicillin allergy in patients requiring β‑lactams), an allergist can perform a graded dose protocol under supervision.
• Alternative medication selection: Choose drugs with no cross‑reactivity; refer to allergy testing results.
• Immunotherapy: Not used for drug hypersensitivity (unlike environmental allergens).

Living with Drug Hypersensitivity Reaction

Medication safety strategies

1. Maintain an updated allergy list: Carry a wallet‑card and ensure the list is in your electronic health record (EHR).
2. Medical alert bracelet: Particularly important for life‑threatening IgE‑mediated allergies.
3. Inform every prescriber: Include the reaction details when new drugs are considered.
4. Pharmacy check: Ask the pharmacist to review prescriptions for cross‑reactive agents.

Self‑monitoring

• Keep a symptom diary when starting a new medication (date, dose, any new rash, breathing changes, etc.).
• Know how to use an epinephrine auto‑injector (EpiPen®, Auvi‑Q®) and carry it at all times if you have a history of anaphylaxis.
• Learn the “3‑P” rule for rash evaluation: Progression, Persistence, Pain. Seek care if rash spreads rapidly, lasts >48 hours, or is painful.

Emotional and social support

Living with drug allergies can cause anxiety about hospital visits or surgeries. Consider:

• Joining patient support groups (e.g., the American Academy of Allergy, Asthma & Immunology – AAACI community).
• Speaking with a mental‑health professional if anxiety interferes with daily life.
• Education for family members on how to respond to an emergency.

Prevention

1. Pre‑prescription screening: Many institutions now require allergy verification before high‑risk drugs (e.g., carbamazepine, allopurinol) are ordered.
2. Pharmacogenomic testing: HLA testing before prescribing abacavir, carbamazepine, or allopurinol in high‑risk ethnic groups reduces severe reactions by >80 % <sup>7</sup>.
3. Avoid cross‑reactive drugs: For β‑lactam allergy, use a non‑β‑lactam class or confirm tolerance through testing.
4. Gradual dose escalation: When possible, start at a low dose and titrate up, especially with antibiotics known for delayed rashes.
5. Vaccination timing: Administer vaccines separate from known triggering drugs to reduce confounding reactions.
6. Patient education: Teach patients to read medication labels, ask pharmacists about inactive ingredients (e.g., latex in vial stoppers), and report any new symptoms promptly.

Complications

If a drug hypersensitivity reaction is not recognized or is inadequately treated, complications can be serious:

• Anaphylactic shock: Can lead to cardiac arrest, respiratory failure, or death within minutes.
• Organ failure: DRESS may cause acute hepatitis, myocarditis, or interstitial nephritis.
• Severe skin loss: SJS/TEN can result in sepsis, fluid‑electrolyte imbalance, and long‑term scarring or blindness.
• Chronic medication avoidance: Inability to use first‑line therapies (e.g., penicillin for streptococcal infection) may lead to suboptimal treatment, longer courses, or use of broader‑spectrum antibiotics that promote resistance.
• Psychological impact: Ongoing anxiety, reduced quality of life, and avoidance of necessary medical care.

When to Seek Emergency Care

If you experience any of the following, call 911 or go to the nearest emergency department immediately:

• Difficulty breathing, wheezing, or a feeling of throat tightness.
• Rapid swelling of the face, lips, tongue, or neck (angio‑edema).
• Sudden drop in blood pressure (light‑headedness, fainting).
• Rapid or irregular heartbeat.
• Severe rash that spreads quickly, blisters, or the skin begins to peel (possible SJS/TEN).
• Persistent vomiting or diarrhea with signs of dehydration.
• Sudden onset of severe abdominal pain, especially with a known drug exposure.

Having an epinephrine auto‑injector on hand and using it at the first sign of anaphylaxis can be lifesaving. After administration, still seek emergency care because a second reaction phase can occur.

References

1. Mayo Clinic. Drug Allergy. Updated 2023. https://www.mayoclinic.org
2. U.S. National Library of Medicine. HLA‑B*57:01 and Abacavir Hypersensitivity. 2022. PMID:35245678
3. World Health Organization. Global Surveillance of Adverse Drug Reactions. 2020. WHO
4. Cleveland Clinic. Drug Allergies and Adverse Reactions. 2024. Cleveland Clinic
5. PharmGKB. HLA‑B*15:02 and carbamazepine-induced Stevens‑Johnson syndrome. 2021. PharmGKB
6. Centers for Disease Control and Prevention. Abacavir Pharmacogenomics. 2023. CDC
7. Van Der Merwe, T. et al. Pharmacogenetics of Severe Cutaneous Adverse Reactions. J Allergy Clin Immunol. 2022;149(3):789‑798.

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