Small red blisters on the arms may be caused by herpes zoster, contact dermatitis, herpes simplex and other diseases. 1. Herpes zoster: Herpes zoster is caused by varicella zoster virus infection, the disease will feel neuralgia, at the same time the skin will appear red pimples as well as blisters, usually along the band or strip distribution. It is usually treated with antiviral drugs, and oral acyclovir or valacyclovir is recommended. 2. Contact dermatitis: If the skin on the arms comes into contact with items to which you are allergic or some of the chemicals, it will cause an allergic reaction in the body, after which blisters will grow on the arms, usually accompanied by itching. It is recommended to use a wet compress with dermatological lotion and apply halometasone cream at the same time. 3. Herpes simplex: Herpes simplex is caused by the herpes simplex virus infection, the main symptom is the appearance of clustered distribution of blisters in the hand area, accompanied by a certain degree of pain. This disease is also oral acyclovir, at the same time can be applied to the local application of Penciclovir ointment for treatment. Daily to keep the arm at the clean and hygiene, do not squeeze the blisters, in order to avoid the phenomenon of infection. At the same time, go to the hospital to clarify the cause of blisters on the arm, targeted treatment.