Allergy to drugs

Drug allergy was rare in the past when the use of medicines was relatively limited. However, in the last 50 years, a huge number of drugs have been developed and their consumption has increased tremendously. As a consequence, drug allergy is much more common today.

Like all other allergic diseases, drug allergy is caused by an inappropriate reaction of the immune system to a chemical substance in the drug. The allergic response can be triggered not only by the drug itself but also by one of the compounds into which the drug is metabolised in the body (most drugs are metabolised).

It is not easy to differentiate true drug-induced allergies from other undesirable effects or abnormal reactions caused by drugs:

• Side effects are unwanted effects of drugs which can be observed at normally recommended doses of many drugs. Examples of side effects include: old generation antihistamines making you drowsy; broad-spectrum antibiotics giving you diarrhoea.
• Overdose effects are toxic effects of drugs which only occur when the recommended dose is exceeded.
• Intolerances are side effects that occur in some people whose body is not capable of metabolising the drugs properly; consequently they cause abnormal reactions.  
• Pseudoallergies manifest very similar symptoms to allergy except that they are not mediated by IgE. A typical pseudo-allergy is intolerance to aspirin and other painkillers which can cause asthma attacks, rhinitis or urticaria and localised swelling (angioedema). Unlike allergy, these attacks may occur the very first time you take the drug (thus ruling out sensitisation).