Conditions and symptoms

Drug-induced hypersensitivity reactions may cause a wide range of symptoms, ranging from mild skin rash to life-threatening anaphylactic shock. The most common are rash and fever, occurring in about 90% of cases, but hypersensitivity can affect any system or organ in the body.

Skin rash can range from mild to severe. The most common is an itchy red measles-like rash that typically develops one to four weeks after starting a new drug. Rashes often appear first on the trunk and then spread outward and they sometimes affect the mucous membranes. While the various types of skin eruptions may look similar, they have different causes and treatments.

Hives or urticaria are itchy raised bumps that may recede in one area and crop up somewhere else. Angioedema is characterised by inflammation and swelling in the deeper levels of skin and subcutaneous tissue, which may involve the lips, eyelids, and genitals, and can cause breathing difficulties if it affects the upper airway. Hives and angioedema are IgE-mediated allergic reactions. Some drugs cause increased sensitivity to light, which may lead to sunburn.

Drugs can also cause more severe skin reactions. Patients with Stevens-Johnson syndrome or erythema multiforme major develop blisters on the skin and mucous membranes. Sections of the skin may die and peel off, leaving raw areas that resemble burns. Other symptoms may include fever, mouth sores and eye inflammation. Toxic epidermal necrolysis is another life-threatening hypersensitivity reaction with similar symptoms, but more extensive skin loss. These are not true allergic reactions, but appear to involve the release of inflammatory cytokines and over-activation of lymphocytes and macrophages.

Fever is a common symptom of drug-induced hypersensitivity. Other possible manifestations include swelling, enlarged lymph nodes, cough, difficulty breathing, sore throat, gastrointestinal symptoms, dizziness, muscle and joint pain, blood-cell deficiencies, blood-vessel inflammation, and liver or kidney problems.

The most severe type of drug-induced allergic reaction is anaphylaxis. This can come on within seconds or minutes after restarting a drug that was used previously. Symptoms may include hives, swelling, constriction of the upper airway, falling blood pressure, rapid heartbeat, shock, and cardiovascular collapse.