Drug Allergy

Pathophysiology

A drug allergy develops when the immune system mistakenly identifies a drug as a harmful substance and attacks it. The immune system activates immune cells produces antibodies and releases histamine, triggering an allergic reaction. Common drugs known to cause allergic reactions include penicillin, non-steroidal anti-inflammatory drugs (NSAIDs), and sulfonamides.

Symptoms

Drug allergies can be classified into immediate allergic reactions and delayed allergic reactions. Immediate allergic reactions usually occur within minutes to hours after exposure to the drug, while delayed allergic reactions may develop days to weeks after drug exposure. The following symptoms typically can appear within a few hours after exposure to the drug:

	Swelling of the lips, tongue, or face
	Itchy eyes
	Wheezing
	Hives
	Rashes or itchy skin

Less common drug allergy reactions include serum sickness (which can present with symptoms similar to infections, such as fever and joint pain), hemolysis, hepatitis, and nephritis.

Anaphylaxis

Anaphylaxis is a severe, life-threatening allergic reaction that can occur within minutes of exposure to an allergen. It can manifest as difficulty in breathing, gastrointestinal symptoms, sudden drop in blood pressure, or even loss of consciousness, and hence requires immediate medical attention and adrenaline injection.

Diagnosing Drug Allergy

Accurate diagnosis relies on detailed medication history and physical examination. In some cases, doctors may recommend a drug challenge test to confirm the allergy; however, due to the risk of severe allergic reactions, this procedure must be performed under strict medical supervision. Skin prick test is another diagnostic tool available for drug allergies but largely limited to penicillin-type drugs.

Treatment

	Once a drug allergy is suspected, the use of the offending drug should be immediately discontinued
	If anaphylaxis occurs, administer adrenaline immediately and seek emergency medical help
	Antihistamines and corticosteroids are commonly used to manage allergic symptoms. Antihistamines can alleviate symptoms such as itching and rashes, while corticosteroids can reduce inflammatory reactions
	For essential drugs with no suitable alternatives, doctors may recommend desensitization therapy. Under medical supervision, the drug is gradually reintroduced to help the immune system adapt and reduce the allergic reaction

Recovery

Preventive Strategies and Long-term Management

	Avoidance of any known allergenic drugs remains the most effective prevention strategy
	Patients should learn to recognize early signs of allergic reactions and seek medical attention promptly when necessary
	For medications known to cause an allergic reaction, inform your doctor and explore any suitable alternative treatments
	Keep a list of current medications and specifically keep note of any past allergic reactions to certain drugs

Myths

1. Are all adverse drug reactions allergies?

Side effects are adverse reactions caused by the chemical properties of the drug itself, whereas allergic reactions are due to an excessive immunological response to the drug. Reactions caused by the chemical properties of the drug are more common than allergic reactions. Accurate diagnosis and testing can help differentiate between the two, enabling the development of suitable treatment plans.

Drug Allergic Reactions

Drug Side Effects

The immune system activates a defense mechanism, identifying the drug as a "foreign substance"

Adverse reactions caused by the chemical properties of the drug itself

2. Does a family history of drug allergy increase personal risk?

A family history of allergic reactions to specific drugs does not increase an individual's risk of experiencing the same drug allergy.

3. I have previously been labelled as allergic to penicillin. Should I undergo a more detailed allergy assessment?

In Hong Kong, research has shown that approximately 1 in 50 individuals are falsely labelled to have penicillin allergies, with nearly 90% of these labels later deemed incorrect. Incorrect drug allergy labels can lead to clinicians avoiding penicillin and related antibiotics, resulting in poorer treatment outcomes, increased mortality, higher healthcare costs, and the development of multi-drug-resistant bacteria. To address this issue, the University of Hong Kong Li Ka Shing Faculty of Medicine established the Hong Kong Drug Allergy Delabelling Initiative (HK-DADI). Under this program, trained nurses and other healthcare professionals triage and assess suspected penicillin allergy cases and help to safely remove false allergy labels. This process enhances penicillin use and improve patient care.