A pityriasis rosea rash lasts between six and eight weeks, says WebMD, but it can last longer. Some people have a rash for months. The rash forms a Christmas tree pattern on the body, notes MedlinePlus.
If the symptoms of the disease are mild, the patient can use home remedies to treat it, says MedlinePlus. These include cool baths, oatmeal baths and applying creams to the skin, including hydrocortisone creams, that soothe the itching or irritation. If the itching is especially bothersome, the patient can also take over-the-counter antihistamines, or a doctor can prescribe corticosteroids. A judicious amount of exposure to the sun or other sources of ultraviolet light may help the rash to heal. The patient should avoid becoming overheated and avoid harsh soaps.
If the doctor is not sure that the rash is pityriasis rosea by looking at it, she may conduct a potassium hydroxide test, states WebMD. To do this, she takes a biopsy of the affected skin and examines it under a microscope. She may also test to see that the rash is not a symptom of syphilis.
Medical professionals are not sure what causes pityriasis rosea, though many believe it is caused by a virus, says WebMD.