Understanding Drug Allergy: Symptoms, Types and Management

Our immune system acts as a protective shield, defending our bodies against bacteria and viruses. When the immune system develops an “excessive sensitivity” to normally harmless substances, it results in an “allergic reaction”. Drug allergy refers explicitly to allergic reactions triggered by the ingestion, topical application, or injection of certain drugs. The immune system recognises certain drugs as “foreign substances” and initiates a defensive or attack response. In general, most drug molecules have small structures that do not capture the attention of the immune system. However, after undergoing metabolism and combining with carrier proteins, a small fraction of drug molecules can form larger molecules that stimulate the body’s immune system, leading to allergic reactions. It’s important to note that drug allergies may not manifest during the initial administration or injection of a drug but can occur upon subsequent exposure after a certain period.

Common symptoms of drug allergies can be classified into two types: acute and delayed.

Acute drug allergy reactions

Acute drug allergy reactions typically occur within one to four hours after receiving an injection or taking medication.

Mild symptoms include:

• Hives (red rashes or urticaria)
• Angioedema (swelling of the face or eyes)
• Nausea, vomiting, and diarrhoea

Severe symptoms include:

• Anaphylaxis
• Difficulty breathing
• Swelling of the mucous membranes in the respiratory tract
• Decreased blood pressure
• Increased heart rate

Delayed drug allergy reactions

Delayed drug allergy reactions typically occur several hours to several days after exposure to the medication and, in some cases, even one to two weeks later. Common symptoms are generally milder, and severe symptoms are less common.

Mild symptoms include:

• Hives (red rashes or urticaria)
• Dermatitis
• Erythematous papules
• Vasculitis

Severe symptoms include:

• Extensive generalised rash, blisters, thickened sores, resembling a burn
• Fever, swollen lymph nodes, affecting the liver, kidneys, and other internal organs
• Stevens-Johnson Syndrome (SJS)
• Toxic epidermal necrolysis (TEN)
• Drug-Related Rash with Eosinophilia and Systemic Symptoms (DRESS)
• Acute generalised exanthematous pustulosis (AGEP)

Here are common drugs that can cause allergic reactions:

1. Antibiotics: Penicillin or cephalosporin drugs
2. Fever reducers: Nonsteroidal anti-inflammatory drugs (NSAIDs) or acetaminophen (paracetamol)
3. Anaesthetics
4. Contrast agents used in medical imaging
5. Chemotherapy drug: Cisplatin
6. Antiepileptic drug: Carbamazepine
7. Uric acid-lowering drug: Allopurinol

No, adverse drug reactions, often referred to as side effects, are distinct from drug allergies. Drug allergies occur when the body’s immune system develops an allergic response to a drug. This type of reaction is unpredictable and specific to individuals with an allergy to that drug. On the other hand, adverse reactions to drugs can occur in anyone and are not necessarily related to the immune system. Side effects are typically a result of the drug’s pharmacological properties, formulation, or dosage. Common examples include nausea, diarrhoea, or headache. Clinical trials and testing of drugs help identify and document these side effects, which are provided in the drug’s package insert for patients to be aware of. If you have any concerns about adverse reactions or suspect a drug allergy, it is advisable to consult healthcare professionals immediately.

The likelihood of experiencing drug allergies is influenced by an individual's immune system and allergic history. Here are some high-risk groups that are more prone to drug allergies:

• Individuals who receive injectable medications are at a higher risk of experiencing severe drug allergy reactions compared to those taking medications orally.
• People with a history of food allergies or allergies to other medications.
• Individuals with chronic illnesses who require long-term use of specific medications, such as antibiotics.

If you suspect that you are experiencing a drug allergy reaction, it is important to take the following steps:

1. Immediately discontinue the use of the suspected medication.
2. Make a note of the allergic reactions you are experiencing.
3. Seek medical attention promptly.

The doctor will assess your allergic reactions, the specific drug involved, and the likelihood of re-administering the drug. Based on these factors, they will determine whether allergy testing is necessary. Skin testing, particularly for antibiotics like penicillin and cephalosporins, is often accurate and carries low risk. These drugs are commonly encountered, so undergoing allergy testing can be valuable.

However, if the drug in question has the potential to affect major organs such as the liver or lungs, doctors generally do not recommend testing. In such cases, they will provide guidance on clearly communicating your allergy history when seeking medical care in the future.

Skin test

Blood test

Drug challenge test

Read more: Drug allergy assessment and test

Virtus Paediatric Immunology, Allergy and Infectious Disease Specialist