Acne on the back of the arms is not uncommon. It could be filled with pus, there could be bleeding on scratching and could leave a scar too.
Homeopathic Medicine has good remedies like Berb aqui, Calendula, Merc sol, Sulph, Calc silicata etc. to get rid of the Acne and prevent further occurrence.
Dr.Venugopal Gouri.