Feline acne develops in the sebaceous glands on the underside of the chin and edges of the lips. Blockage of skin pores by excess sebum or keratin is a predisposing cause. It is more common in cats with oily skin. It is not analogous to acne in people.
Feline acne is identified by finding blackheads or pimplelike bumps that come to a head and drain pus. Swelling of the entire chin and lower lips may be seen in severe cases.
Brushing your cat not only removes dirt, grease and dead hair from her coat, but it helps to remove skin flakes and stimulates blood circulation, improving the overall condition of her skin. One or two brushings per week will help kitty to keep her healthy glow and allow her to bask in yummy together time-and you’ll find that regular sessions are especially beneficial when your cat ages and is no longer able to groom so meticulously on her own.
A similar condition can be caused by an allergic reaction to rubber or plastic food and water bowls. In those cases, simply changing to stainless steel or ceramic dishes will clear up the condition.
Treatment: The infection usually responds to cleansing of the skin twice a day with an ointment or gel containing 2.5 to 5 percent benzoyl peroxide (OxyDex), chlorhexidine (Nolvasan) or povidone-iodine (Betadine). When excess sebum is a factor, the skin should be cleansed with a medicated shampoo for cats. An extensive or deep infection may require antibiotics. Because the underlying problem remains the same, acne often recurs when treatment is stopped. Some cats do better if switched from a wet to a dry food, or if the owner cleans their chin after every meal.
WebMD Veterinary Reference from "Cat Owner's Home Veterinary Handbook"