A lot of small red dots on the arm may be caused by eczema, purpura, urticaria and other diseases, which can be treated by oral medication and application of medication. 1. Eczema: is a variety of causes of superficial dermis and epidermal inflammation, the disease can lead to skin parts of the pinhead size or the size of the corn size of the papules, which will cause a lot of small red dots on the arm. You can apply glycerine lotion, hydrocortisone butyrate cream and other medications. 2. Purpura: is the red blood cells from the blood vessels to the skin, connective tissue or mucous membrane exudation caused by the disease, the skin color can become purple, brownish-red, and so on, so it will cause a lot of arm reddish spots. You can take oral rutin tablets, vitamin C tablets and other medications. 3. Urticaria: a common skin disease, the disease will lead to the appearance of skin size, shape or red spots, so that the patient appeared to arm a lot of small red spots phenomenon. You can take levocetirizine hydrochloride tablets, loratadine tablets and other drugs. Arms with a lot of small red spots may also be caused by insect bite dermatitis, the patient needs to go to the regular hospital to clarify the cause of the disease, and then under the guidance of the doctor to use medication.