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:
Tip: sometimes when the same allergic reaction occurs with a large number of drugs which are not chemically related, the cause might be one of the inert compounds common to all the drugs.
In a drug allergy, there will always be IgE antibodies that react with the drug or its breakdown products (metabolites). IgE antibodies are absent in all other drug adverse reactions, including pseudoallergies.