Treatment for a rash caused by an allergic reaction may require a prescription drug. Over-the-counter medications and topical preparations can be used. A rash accompanied by breathing difficulty is a medical emergency. Identifying the source of a rash is important. Some bacterial and viral infections can cause a rash that mimics an allergic reaction.
Antihistamines sold over the counter can reduce inflammation and calm the itching from an allergic reaction. Examples include diphenydramine, or Benadryl; loratadine; and certrizine. Diphenhydramine can cause drowsiness and would best be taken at bedtime.
Cortisone creams can be purchased at the drug store and applied topically to treat a rash caused by an allergic reaction. According to the Merck Manual, diphenhydramine applied to the skin should be avoided because it could also cause an allergic reaction. Hydrocortisone cream should be applied to small areas because it can be absorbed into the bloodstream and cause medical problems, especially in infants. Facial rashes should be treated with no stronger than 1 percent hydrocortisone cream to avoid skin damage.
Calamine lotion applied after bathing in cool or lukewarm water can soothe a rash caused by an allergic reaction. The skin should be dried thoroughly before applying. If the rash is no better in 7 days, a physician's exam is recommended. Calamine lotion can be applied three to four times daily.
Colloidal oatmeal lotions and baths calm itching from allergies that causes rashes. Soaking in a tub of cool water or applying compresses soaked in colloidal oatmeal can be especially effective for rash caused by poison ivy. Several preparations that can calm inflammation and itching are available, including lotions, powders, shampoos and shaving gels.
A physician might prescribe oral corticosteroids for a rash that persists or is widespread. A common example is prednisone, which can be taken short term in tapering doses. Corticosteroids mimic the effect of natural steroids produced by the adrenal glands. They are used for a variety of health problems, including asthma and arthritis, as well as allergies.