[Allergic reaction to topical drugs]

It is important to differentiate worsening of a cutaneous eruption following topical therapy from lack of response to the medication. A study to determine the origin of contact dermatitis secondary to topical therapy was conducted among 87 patients with this kind of contact dermatitis. Sixteen patch tests with chemical compounds known to be allergenic were done. The following eight chemical compounds were responsible for 86.5% of the positive reactions observed: neomycin, ethylenediamine, mercury, benzocaine, iodochlorhydroxyquin, peruvian balsam, nitrofurazone and cinchocaine. More than half of the reactions were to neomycin, ethylenediamine and mercury. It is easier to diagnose this condition if one knows the composition of the topical medications one prescribes and the allergenic potency of their ingredients.