Overview
Drug allergy is an immune-mediated adverse reaction that occurs when the body mistakenly identifies a medication as a harmful substance. In affected individuals, the immune system produces immunoglobulin E antibodies against the drug or its metabolites, triggering the release of chemical mediators that cause allergic reactions.
These reactions may occur within minutes to a few hours after drug exposure or may appear days to weeks later, depending on the immune mechanism involved. Drug allergy testing measures the immune response to specific medications and helps identify the causative drug. Accurate diagnosis is essential to prevent re-exposure, ensure patient safety, and guide the selection of alternative medications
Symptoms
Symptoms of drug allergy range from mild to severe and can be classified as early or late reactions. Early symptoms commonly include skin rash, hives, itching, fever, and swelling of the face, lips, or tongue. Respiratory symptoms such as wheezing, shortness of breath, runny nose, itchy or watery eyes, and chest tightness may occur. Severe immediate reactions can progress to anaphylaxis, characterized by difficulty breathing, hypotension, dizziness, and loss of consciousness.
Late reactions include serum sickness, presenting with fever, joint pain, rash, swelling, and nausea. Drug-induced anemia may cause fatigue and breathlessness. Severe delayed reactions such as drug reaction with eosinophilia and systemic symptoms present with high fever, widespread rash, facial edema, lymphadenopathy, hematological abnormalities, and involvement of organs including liver, kidneys, heart, lungs, and endocrine system. Blistering skin disorders such as Stevens–Johnson syndrome and toxic epidermal necrolysis represent life-threatening manifestations.
Causes
Drug allergy is caused by abnormal immune responses to medications. IgE-mediated reactions result in immediate hypersensitivity through mast cell degranulation.
Cytotoxic antibody-mediated reactions damage blood cells, leading to anemia or thrombocytopenia. Immune complex-mediated reactions cause inflammation and vasculitis. T-cell-mediated delayed reactions result in contact dermatitis and severe cutaneous adverse reactions. Non-immunologic reactions may mimic allergy but occur without immune involvement. Common drug groups associated with allergic reactions include antibiotics, antiepileptics, antitubercular agents, nonsteroidal anti-inflammatory drugs, analgesics, and certain cardiovascular and psychiatric medications.
Risk Factors
Risk factors for drug allergy include previous exposure to the same or structurally related drugs. A personal or family history of allergy, asthma, or atopic conditions increases susceptibility. Genetic factors affecting immune response and drug metabolism contribute to hypersensitivity risk. Repeated or prolonged drug use increases the likelihood of sensitization. Underlying viral infections, immune disorders, and the use of multiple medications further raise risk. Older age, female sex, and the presence of chronic illnesses are also associated with increased incidence of adverse drug reactions.
Prevention
Prevention of drug allergy focuses on identification and strict avoidance of the offending medication. Patients should inform all healthcare providers about known drug allergies and maintain an updated list of allergic reactions. Wearing a medical alert bracelet or carrying a drug allergy card helps prevent accidental exposure. Drug allergy testing supports confirmation of true allergy and assists in selecting safe alternatives. Patients with a history of severe reactions should carry emergency medications such as antihistamines or epinephrine as prescribed.
Drug desensitization should be performed only under specialist supervision when no alternative therapy is available. Early recognition, accurate documentation, and patient education are key to preventing recurrent and severe drug-related allergic reactions.
