Allergic rash generally refers to urticaria or contact dermatitis, urticaria usually subside within 24 hours, contact dermatitis usually in the removal of allergens will subside in about 1 to 2 weeks, serious cases may not subside, need to consult a doctor in time, active treatment. 1. Urticaria: common causes include food, infection, drugs, respiratory tract inhalants and skin contacts, etc. The lesions are reddish masses of varying sizes, round, oval or irregular, and the lesions usually last no more than 24 hours. Urticaria has the risk of asphyxiation, so it is necessary to consult a doctor in a timely manner, and not to wait for the lesions to subside on their own. 2. Contact dermatitis: lesions are mostly confined to the contact area, a few can spread or involve the surrounding areas. Typical lesions are clearly defined erythema with papules and papules on top, which can be cured within 1 to 2 weeks after removing the contact material and after active treatment, while improper treatment in some patients may lead to recurrent episodes that cannot subside on their own. Allergic rashes need to be treated in a timely manner in a hospital, under the guidance of medical professionals to standardize the treatment, in order to eliminate the rash faster.