Drug allergies

Drug allergy: When medications trigger an immune response

Drug allergy is an unpredictable reaction of the immune system to a medical drug. It is important to distinguish true allergy from a drug side effect. A side effect is a known, expected and often dose‑dependent effect of the drug (e.g., nausea from antibiotics). An allergy, however, is an immune system reaction, can occur at any dose of the drug and does not depend on its pharmacological properties.

Mechanism of drug allergy

The immune system recognizes the drug molecule (or its metabolic product in the body) as a foreign and dangerous substance (antigen). In response, it initiates a defensive mechanism that can be of two types:

• Immediate reaction (IgE-mediated): Develops within an hour after taking the drug. This is the classic type allergic reaction, accompanied by histamine release and can manifest as hives, Quincke edema or anaphylaxis.
• Delayed reaction (T-cell mediated): Develops over several hours or even days. Most often appears as various skin rashes.

Symptoms and the most common “culprit” drugs

The most common manifestation of drug allergy is a skin rash. However, reactions can also be systemic, affecting internal organs. The most dangerous manifestation is anaphylactic shock. While any drug can cause an allergy, there are groups of drugs that do so most frequently:

• Antibiotics: Especially the penicillin class and sulfonamides.
• Non‑steroidal anti‑inflammatory drugs (NSAIDs): Aspirin, ibuprofen.
• Anesthetics, used in surgical procedures.
• Chemotherapeutic agents.

Any suspicion of a drug allergy requires immediate discontinuation of the drug and consultation with a physician.