Allergic contact dermatitis from a shampoo?

Q. My doctor says the rash on my scalp and neck is allergic contact dermatitis from a shampoo I’ve used for several weeks. Why didn’t the rash appear the first time I used the shampoo?

A. It takes more than one exposure to an allergen for an allergic reaction to occur in susceptible people. (An allergen is a substance responsible for an allergic reaction.) You may have used this shampoo quite a few times before you developed a sensitivity to it and broke out in a rash.

Now that you are sensitized to it, you will develop the rash of allergic contact dermatitis every time you use that product.

It may not be the shampoo itself that brings on the blisters and bumps. Rather, it could be the perfume that is in the shampoo, creating the pleasing fragrance.

Other common causes of allergic contact dermatitis are gold and nickel. Nickel is found in jewelry as well as in zippers and buttons. Two antibiotic ointments—bacitracin and neomycin sulfate—are common allergy culprits as well.

Allergic contact dermatitis isn’t the same as irritant contact dermatitis, which occurs when a substance injures the skin. Sometimes it’s a harsh chemical doing the damage, but milder detergents, soaps, and even water can prompt a skin reaction. Both allergic contact dermatitis and irritant contact dermatitis may show up on the hands. But a rash on the scalp, eyelids, ears, lips, neck, or underarms is probably an allergic reaction.

Skin creams that contain hydrocortisone can relieve the itching of allergic contact dermatitis, but oral medications might be needed to quell a strong allergic reaction. You’ve probably stopped using the shampoo in question, which is a good move. As with any other kind of allergy, avoiding the allergen is the first step in controlling allergic reactions.

Questions are selected that reflect the wide variety of health problems of general interest. We cannot, however, respond specifically to the personal health concerns of an individual.