According to research published early online in the British Journal of Dermatology, medicines may be wrongly blamed for cutaneous reactions in around one quarter of cases.

The authors note that skin reactions are one of the most common side-effects of medications (usually related to drug hypersensitivity), occurring in 2-3% of medical patients.  Determining causality can be difficult as the clinical picture often gives no clue to the offending drug; patients are often taking more than one drug concomitantly and have a medical condition, so the reaction may also be reactive.  Although previous studies have investigated drug-induced cutaneous reactions in hospital patients, few have extensively tested patients to prove diagnosis. 

The current study reports on a total of 612 patients who were referred to a tertiary care hospital in Switzerland with suspected cutaneous drug reactions.  A total of 141 of these patients were suspected by an allergologist to have had a cutaneous drug reaction and were invited for further assessment to identify the culprit drug, with evaluation by allergological testing (e.g. prick/ intracutaneous testing and scratch/patch) and drug provocation testing if necessary.   
 

A drug was identified as the cause in 107 of these cases (76%) whereas 34 (24%) were reactive rashes or had other causes.  The most common types of reaction were maculopapular rashes and urticaria/angio-oedema.  The most commonly implicated drugs included antibiotics (39.8%) followed by anti-inflammatories (21.2%), contrast media (7.6%) and others (31.4%; including oral antidiabetics, antimycotics, antipsychotics, antiepileptics, for example).

The authors conclude that clinical assessment overestimates the role of drug allergies in cutaneous reactions, and comment that it is essential to conduct allergological testing in patients presenting with suspected cutaneous adverse drug reactions, in order to avoid the exclusion of drugs for patients who are not allergic to them.