Contact rashes are typically red, scaly, itchy, and blistering. Although linking the outbreak of a rash to the use of a new cosmetic can be a simple matter when only a few cosmetics have been used, it may become difficult if you routinely use a wide variety of different cosmetics.

Finding the culprit can be even more difficult for your dermatologist when the allergic rash appears far from the actual site of application of the cosmetic. For example, you may be surprised to learn that nail polishes are responsible for many instances of allergic reactions around the eyes and eyelids (because the allergens are transferred there by your fingers). Similarly, nail polish may also be the cause of allergies on the legs if you use it to repair stocking runs. (Of course, allergies occur occasionally around the nails as well.) And allergies to shampoos or hair sprays typically show up as rashes on the neck, upper back, or forehead rather than the scalp, which is actually seldom affected by them. Considering the foregoing, a keen attention to detail is frequently required in order to make the correct diagnosis and isolate the real troublemaker(s).

When one or two cosmetics have been singled out as possible troublemakers, your doctor may suggest patch testing placing a small amount of the suspected cosmetic on a patch and applying it to the skin of the inner upper arm or back to see whether the allergic rash is reproduced. If there is a reaction, your doctor may then contact the particular cosmetic manufacturer to obtain test amounts of the individual ingredients in the cosmetic for further patch testing. In this way the exact ingredient(s) that causes your allergy problem can be determined, and you can avoid purchasing all products that contain it in the future. On the other hand, when a cosmetic allergy is suspected but no particular cosmetic can be singled out for blame, your doctor may recommend patch-testing you to all your cosmetics or to a standard screening tray that is composed of some of the more common allergens found in most cosmetics.

Regardless of which method is used, once the allergy-provoking ingredients are determined, your doctor can provide you with a list of products that ordinarily contain them and advise you as to what alternatives (if any) exist and what other ingredients must also be avoided because they are chemically related to the ingredients to which you have been found allergic.