If a change in a skin care product does not lead to clearing of a patient’s rash, does this mean that the original product was not the culprit?

Not necessarily. Many consumer cosmetic and toiletry products contain the same allergens (usually fragrances and preservatives). Moreover, many products contain cross-reacting agents that can exacerbate the original problem. For example, patients who are allergic to the hair dye allergen paraphenylenediamine will need to avoid the over-thecounter topical anesthetic benzocaine. Both compounds belong to the para-amino group and can cross-react with one another.