Acne is a common inflammatory skin disease characterized by pimples on the face, chest, and back. It occurs when the pores of the skin become clogged with oil, dead skin cells, and/or bacteria.

Acne vulgaris, the medical term for common acne, is the most common skin disease. It affects nearly 17 million people in the United States. While acne can occur at any age, it usually begins at puberty and worsens during adolescence. Nearly 85% of people develop acne some time between the ages of 12 and 25 years old.

Up to 20% of women develop mild acne. It is also found in some newborns. The sebaceous glands lie just beneath the skin’s surface. They produce sebum, an oily secretion that helps to preserve the flexibility of the hair and moisturizes the skin. These glands and the hair follicles within which they are found are called sebaceous follicles.

These follicles open onto the skin through pores that allow the sebum to reach the hair shaft and the skin. In certain situations, the glands excrete excess sebum that cannot be cleared from the pores efficiently. This excess happens, for instance, at puberty when increased levels of the androgen hormones cause overproduction of sebum.

In addition, cells lining the follicle are shed too quickly and begin to clump together. The excess sebum combines with the dead cells and forms a plug, or comedo (also called comedones), which is not usually seen, that blocks the pore.

When the follicle begins to bulge and show up as a small whitish bump mostly under the skin, it is called a whitehead. If the comedo opens up, the top surface of the plug darkens, and it is referred to as a blackhead.

Infection results when a plugged follicle is invaded by Propionibacterium acnes, a bacterium that normally lives on the skin, and possibly other microorganisms. The bacterium produces chemicals and enzymes that bring on inflammation.

Pimples are the result of infected blackheads or whiteheads that rupture, releasing sebum, bacteria, dead skin, and white blood cells onto the surrounding tissues. Inflamed pimples near the skin’s surface are called papules; they are red and raised and may be quite tender to the touch.

The papules may become filled with pus and are then called pustules. If the follicle continues to enlarge rather than rupture, it forms a closed sac, called a cyst, which can be felt as a lump under the skin. Large hard swellings deep within the skin are called nodules. Both nodules and cysts may cause pain and scarring.

Acne has a characteristic appearance and is, therefore, not difficult to diagnose. A complete medical history should be taken, including questions about skin care, diet, factors that improve or worsen the condition, medication use, and prior treatment. Physical examination includes the face, upper neck, chest, shoulders, back, and other affected areas.

Under good lighting, the doctor can determine what types and how many blemishes are present, whether they are inflamed, whether they are deep or superficial, and whether there is scarring or skin discoloration.

Blood tests are done when the patient appears to have hormonal or other medical problems. Stool tests can be helpful in determining whether there is a bacterial or yeast overgrowth contributing to the condition. Food allergy testing should also be considered.

Most dermatologists use a combination of therapies to treat acne, depending on the individual. Results of specific treatments vary. Acne is not a serious health threat. The most troubling aspects of this condition are the negative cosmetic effects and potential for permanent scarring.

Some people, especially teenagers, become emotionally upset about their condition, and this psychological aspect may contribute to social or other emotional problems. Acne is not considered curable, although it can be controlled by proper treatment, with improvement possibly taking many months.

Acne tends to reappear when treatment stops, but it often spontaneously improves over time. Inflammatory acne may leave scars that require further treatment.