Primarily the liver. One of the roles of the liver is to break down excess hormones, such as androgens that contribute to acne. This is why it is best treated internally rather than topically.

Digestive bitters work wonders for Acne because they stimulate flushing of the liver allowing the liver to do its job more efficiently. Recommended dose is 1/2 dropper full 1-2 times daily on the tongue right before meals. Bitters will stimulate the bitter receptors on the tongue stimulating the vagus nerve. This in turn flushes the liver. Drink a lot of water throughout the day when using bitters. They are sold in health food stores.

Zinc, 50mg a day with a large meal to prevent nausea, is also recommended to help control androgens.