Drug Allergy — Symptoms, Causes & Treatment | MyMedicPlus

A drug allergy is an adverse, immune-mediated reaction to a medication — distinct from predictable pharmacological side effects, drug toxicity, or intolerance. True drug allergies account for 5-10% of all adverse drug reactions, though approximately 10% of the general population report a drug allergy — the discrepancy reflects that many reactions attributed to allergy are actually pharmacological side effects or intolerances that do not involve the immune system. Drug allergy encompasses a clinical spectrum from mild skin rashes to immediately life-threatening anaphylaxis. Penicillin and related beta-lactam antibiotics are the most commonly reported drug allergen class, accounting for up to one-third of all reported drug allergies. Critically, up to 80-90% of patients labelled as 'penicillin allergic' can safely tolerate penicillin when formally evaluated — removal of inaccurate allergy labels significantly improves antibiotic stewardship and patient access to optimal treatment. Other common culprits include NSAIDs, sulfonamides, anticonvulsants, and iodinated contrast media. All healthcare professionals should maintain accurate drug allergy records to avoid unnecessary drug avoidance and ensure safe prescribing.

Drug allergies are classified by the Gell-Coombs immunological mechanism. Type I (IgE-mediated, immediate hypersensitivity): preformed IgE antibodies bind drug-hapten conjugates on mast cells and basophils, triggering degranulation within 1-6 hours — causing urticaria, angioedema, bronchospasm, or anaphylaxis; penicillin anaphylaxis follows this mechanism. Type II (cytotoxic, antibody-mediated): antibodies target drug-hapten conjugates on cell surfaces — causing drug-induced haemolytic anaemia (methyldopa, penicillin), thrombocytopaenia (quinine, heparin), or neutropaenia. Type III (immune complex): drug-antibody complexes deposit in tissues causing serum sickness (fever, rash, arthralgia 7-21 days post-exposure — amoxicillin, cefaclor) and drug-induced vasculitis. Type IV (T-cell mediated, delayed hypersensitivity): T-lymphocyte activation occurs 24-72 hours to weeks after drug exposure — causing maculopapular exanthem, contact dermatitis, DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms — onset 2-8 weeks), Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (TEN). Commonly implicated drugs: penicillins and cephalosporins, sulfonamides (trimethoprim-sulfamethoxazole), NSAIDs and aspirin, contrast media, anticonvulsants (carbamazepine, phenytoin, lamotrigine — SJS/TEN risk, especially in HLA-B*15:02 Asian carriers), allopurinol (HLA-B*58:01 in Han Chinese), and biologics. Risk factors: prior drug reaction, atopic constitution, high-dose or parenteral drug exposure, immunosuppression.

Drug allergy reactions span a wide clinical spectrum by timing and severity. Immediate reactions (within 1-6 hours): urticaria (itchy wheals), flushing, nasal congestion, vomiting; angioedema (subcutaneous swelling of face, lips, tongue — potentially life-threatening if laryngeal); anaphylaxis (multisystem: bronchospasm causing wheeze and stridor, hypotension from vasodilatation, urticaria, tachycardia, collapse, loss of consciousness). Delayed reactions (hours to weeks): maculopapular exanthem (flat or slightly raised pink-red rash appearing 7-14 days into a course — most common drug rash; amoxicillin, ampicillin); fixed drug eruption (recurring lesion at the same skin site on each re-exposure); photosensitivity eruption; serum sickness (fever, rash, arthralgia, lymphadenopathy 1-3 weeks post-exposure). Severe cutaneous adverse reactions (SCARs): Stevens-Johnson syndrome (SJS — epidermal detachment below 10% body surface area with painful mucosal involvement and targetoid lesions progressing to blistering); toxic epidermal necrolysis (TEN — detachment above 30% BSA, mortality 30-40% from sepsis and multi-organ failure); DRESS/drug hypersensitivity syndrome (morbilliform rash, fever above 38°C, lymphadenopathy, eosinophilia above 1.5 × 10⁹/L, and hepatitis/nephritis/pneumonitis — onset 2-8 weeks after drug initiation; anticonvulsants and allopurinol are the most commonly implicated drugs).

Accurate drug allergy diagnosis begins with a structured clinical history: the exact drug name, dose, route, and duration of treatment; time from administration to reaction onset; full description of symptoms and signs (urticaria, angioedema, anaphylaxis, rash morphology, mucosal involvement); resolution on drug withdrawal; and prior tolerance of the same or chemically related drugs. Immediate reactions: skin prick testing (SPT) to native drug (validated for penicilloyl-polylysine, penicillin G, amoxicillin — sensitivity approximately 70%); intradermal testing (IDT) is more sensitive than SPT; serum tryptase (collected 1-3 hours after reaction; above 11.4 µg/L confirms systemic mast cell activation consistent with anaphylaxis); drug-specific IgE (ImmunoCAP — available for penicillin major determinant, some platinum drugs — negative result does not safely exclude allergy). Delayed reactions: patch testing at 48 and 96 hours. Drug provocation challenge (DPC — graded dose challenge): the gold standard for confirming or safely excluding drug allergy — incremental drug doses administered at 30-60 minute intervals under close medical supervision with resuscitation immediately available; essential to de-label inaccurate penicillin allergy (80-90% of labelled patients tolerate challenge). Pharmacogenomic testing: HLA-B*57:01 before abacavir (mandatory in HIV); HLA-B*15:02 before carbamazepine/lamotrigine in Southeast and South Asian patients; HLA-B*58:01 before allopurinol in Han Chinese patients.

Immediate management: stop the causative drug immediately; clearly document the reaction in all medical records with drug name, date, and reaction type. Mild reactions (urticaria, maculopapular rash without systemic features): oral cetirizine 10mg daily or chlorphenamine 4mg three times daily; short course of oral prednisolone 30-40mg daily for 5-7 days for widespread urticaria; topical hydrocortisone 1% cream for localised rashes. Moderate angioedema: IV chlorphenamine 10mg plus IV hydrocortisone 200mg; if laryngeal involvement is suspected — immediate adrenaline (epinephrine) 500 micrograms IM plus urgent airway assessment. Anaphylaxis (life-threatening emergency): adrenaline (epinephrine) 500 micrograms (0.5 mL of 1:1,000 solution) IM into the outer mid-thigh — the single most important intervention; call emergency services; position patient supine with legs elevated if hypotensive; high-flow oxygen 15 L/min; IV access and 500-1,000 mL crystalloid bolus; IV chlorphenamine 10mg and IV hydrocortisone 200mg after adrenaline; repeat adrenaline at 5-minute intervals if no improvement; observe minimum 6-24 hours after recovery (biphasic reaction risk in 5-20%). Discharge: prescribe two adrenaline auto-injectors (EpiPen 0.3 mg in adults); refer to allergy/immunology. Severe cutaneous adverse reactions (SJS, TEN, DRESS): admit to dermatology or burns unit urgently; stop causative drug; IV fluid management, skin care, ophthalmology review; IV immunoglobulin or ciclosporin for SJS/TEN in selected cases. Drug desensitisation: standardised graded dose protocols allow administration of an essential drug (penicillin, aspirin, platinum chemotherapy) despite allergy — performed under specialist supervision in an appropriate clinical setting.

Fatal anaphylaxis: penicillin causes approximately 1 death per 5-10 million prescriptions — death results from asphyxia (laryngeal oedema, bronchospasm) or refractory cardiovascular collapse; risk is highest when adrenaline is not administered within minutes. Biphasic anaphylaxis: a second wave of anaphylaxis occurring 1-72 hours after apparent complete recovery, in up to 5-20% of cases — necessitating hospital observation for a minimum of 6-24 hours. Toxic epidermal necrolysis (TEN): 30-40% mortality from septicaemia (loss of skin barrier), fluid imbalance, and multi-organ failure; long-term sequelae include scarring, chronic sicca syndrome, corneal scarring causing blindness (symblepharon, entropion), and mucosal strictures (oesophageal, urogenital). DRESS syndrome: fulminant hepatic failure (rare but fatal); AKI requiring renal replacement therapy; myocarditis; prolonged immunological dysregulation requiring months of immunosuppression after drug withdrawal. Quality of care consequences of inaccurate allergy labelling: patients labelled 'penicillin allergic' are 23% more likely to receive suboptimal broad-spectrum antibiotic alternatives — associated with higher rates of Clostridioides difficile infection, MRSA, treatment failure for serious infections, increased antimicrobial resistance, longer hospital stays, and increased healthcare costs.

Document all drug allergies clearly and prominently in every medical record, electronic prescribing system, hospital EHR, and discharge letter — include the drug name, precise reaction type, and date of occurrence. Ensure the patient carries a written allergy list; prescribe a MedicAlert bracelet or equivalent for confirmed anaphylaxis. Prescribe two adrenaline auto-injectors (EpiPen) to all patients with a history of drug-induced anaphylaxis — train both patient and carers to use them. Communicate all drug allergies verbally to all treating clinicians (GPs, hospital doctors, dentists, pharmacists, paramedics) at every clinical encounter. Formal allergy evaluation: refer all patients with penicillin allergy labels for skin testing and drug provocation challenge — 80-90% will be de-labelled, enabling optimal antibiotic prescribing, reducing C. difficile risk, and improving infection outcomes. Drug desensitisation can safely enable administration of a necessary drug (penicillin, aspirin, platinum chemotherapy, biologic agents) despite confirmed allergy — a specialised procedure performed under continuous monitoring in an appropriate clinical setting. Pharmacogenomic screening before high-risk prescribing: HLA-B*57:01 before abacavir (HIV patients); HLA-B*15:02 before carbamazepine or lamotrigine in Southeast and South Asian patients; HLA-B*58:01 before allopurinol in Han Chinese patients — prevents severe, sometimes fatal hypersensitivity reactions.

Call emergency services immediately for any signs of anaphylaxis after taking a medication: throat tightening, difficulty breathing or swallowing, widespread hives with dizziness, hypotension, collapse, or loss of consciousness. Administer adrenaline auto-injector (EpiPen) immediately if prescribed. See a doctor urgently for: severe skin rash after starting a new medication — particularly widespread blistering (Stevens-Johnson syndrome/TEN), facial swelling without urticaria (angioedema from ACE inhibitor), or a skin rash with systemic features (fever, eosinophilia, organ dysfunction — DRESS syndrome). See your GP to document drug allergies accurately, obtain allergy testing through an allergist, wear a MedicAlert bracelet or ID for serious drug allergies, and ensure your allergy is recorded in all your medical records. Always inform every healthcare provider (including dentists) of your known drug allergies.