Blackheads, otherwise known as comedonal acne, are caused by plugging of the pores. In order to unplug the pores and prevent this from occurring, dermatologists often prescribe a group of medications called retinoids. These are usually applied in a small amount at bedtime and are excellent at clearing the comedones.
Also see Acne entry above and our article: What Causes Acne?
For more information, visit the American Academy of Dermatology at www.aad.org/skin-conditions