How can oral medication for acne know how to clear up the area?

Whole body. When a systemic antibiotic changes the body's flora, the effects will be seen where bacteria are already a problem. When a systemic vitamin-a analogue makes the sebaceous glands regress, the effects will be seen where they are actually causing acne, but the rest of the skin will be affected and dry up as well. This is common to medications in general, even though they seem magic.