Simply put, a drug eruption is a skin eruption caused by a medication or drug. So is it appropriate to use the term “drug eruption” alone clinically and/or histologically? A strong case can be made that the answer should be “no.”

An eruption due to a medication may mimic most primary skin eruptions, e.g., urticaria, erythema multiforme (EM), pityriasis rosea (PR), psoriasis, panniculitis , bullous disorder, lichen planus (LP) , dermatitis, and more. Hence, the term drug eruption should always be associated with the morphological type of the eruption. For example, urticarial drug eruption, pityriasis rosea-like drug eruption , drug-induced erythema multiforme, and eczematous drug eruption, among many others.

The histopathology of each of these drug eruptions is generally similar to that of the primary eruption, whether due to herpes virus infection for erythema multiforme, food allergy for urticaria, or systemic fungal infection for panniculitis.

Some histological features may raise suspicion that an eruption may be due to a medication rather than idiopathic or due to other causes. None of these features, however, are diagnostic of drug etiology.

The histological characteristics of LP, for example, are well known (compact orthohyperkeratosis, hypergranulosis, saw-tooth appearance of the rete, and a usually dense band-like lymphocytic infiltrate in the papillary dermis that obscures the epidermal basal layer and dermal epidermal junction). Drug-induced lichen planus (or lichen planus-like drug eruption) may have identical features to idiopathic lichen planus but may have foci of parakeratosis and some eosinophils among the lymphocytes. This conclusion was drawn from comparing biopsy specimens from a group of patients with idiopathic lichen planus to a group with drug-induced lichen planus. The correlation between the medication etiology and the two histological features was not absolute. Instead, there was an overlap in the histological findings between idiopathic and drug-induced LP.