Acne is the most common skin disorder in adolescents. Most teens in Western countries experience some degree of acne, which generally resolves as androgen levels decline. Some cases, however, persist into adulthood or have their onset in adulthood.
Common acne, as it appears in adolescents, is a disease of pilosebaceous follicles. Follicular h yperkeratinization, increased sebum production, overgrowth of P ropionibacterium acnes bacteria, and subsequent inflammation contribute to the clinical manifestation of acne. Plugs founds in acne follicles are composed of keratin, from surrounding keratinocytes, and lipids, secreted from sebaceous glans under the influence of androgenic hormones. The lipid-rich sebum serves as a medium for proliferation of P acnes.
Acne may appear as closed comedones (“whiteheads”) or, when excessive distention forces open the follicular orifice, open comedones (“blackheads”). Oxidation of the lipid and melanin cause the darkened appearance of open comedones.
Acne most commonly affects areas of the body with the greatest number of sebaceous glands. These include the face, upper back, neck, chest, and upper arms. Mild acne is not inflammatory and involves only a small number of open or closed comedones. Moderate-to-severe acne involves inflammation of the dermis surrounding the pilosebaceous unit. This inflammation is due to follicular rupture, spilling of free fatty acids (resulting from the hydrolysis of triglycerides by P acnes), and the release of lysosomal enzymes from neutrophils attracted by chemotactic factors released by P acnes. This process results in the formation of pustules, papules, or nodules that cover a large area of skin. Scarring and hyperpigmentation can also occur, which is more common in patients with darker complexions.
Cosmetics (acne cosmetic). Skin and hair products that contain oils or dyes can exacerbate acne lesions. Water-based cosmetics are less comedogenic.
Repetitive skin trauma (acne mechanica). Rubbing (even with cleansing agents), scrubbing, or occlusive clothing (e.g., bra straps, helmets, turtlenecks) can promote inflammatory reactions in the lesions.
Environmental exposures. Humidity and sweating can exacerbate acne. Exposure to certain chemicals (e.g., dioxin and other halogenated hydrocarbons) that are found in herbicides and other industrial products can cause severe inflammatory acne and scarring.
Drugs. Certain drugs are likely to cause acne, including corticosteroids, phenytoin, isoniazid, disulfiram, lithium, and B vitamins.
Diet. Milk intake, in particular, has been linked to acne (see Nutritional Considerations below).
Genetics. Genetics may play a role in more severe forms of acne, especially in persistent and late-onset cases. The role of genes in mild adolescent acne is uncertain.
Stress. Stress is believed to be associated with acne exacerbations, but further study is required to establish this connection.
Hormones. Endocrine disorders marked by excess androgens or insulin resistance, such as congenital adrenal hyperplasia and polycystic ovarian syndrome, may trigger the development of acne vulgaris.
Acne vulgaris is a clinical diagnosis. History and dermatologic examination are necessary to characterize the distribution and types of acne lesions and to evaluate underlying medical disorders.
Fever and arthralgia in a patient with severe inflammatory acne suggest acne fulminans, a serious disease that requires immediate treatment with systemic corticosteroids and isotretinoin (see below).
Women with oligomenorrhea, hirsutism, male- or female-pattern alopecia, infertility, acanthosis nigricans, and truncal obesity should be evaluated for hyperandrogenism, which may occur due to polycystic ovarian syndrome or an androgen-secreting tumor.
Treatment should address both the physical and psychological effects of acne and should be guided by the severity and type of lesions. Light and laser therapies may be used but there is a paucity of significant clinical evidence demonstrating benefit at this time.
A number of topical therapies are used to treat noninflammatory acne.
Retinoids (e.g., tretinoin, adapalene, tazarotene) decrease follicle hyperkeratinization and are commonly recommended as part of treatment for any degree of acne. Retinoids also accelerate the resolution of hyperpigmentation caused by acne induced inflammation. They are available in various preparations, including creams, gels and microgels (which are less irritating), solutions, and pads. Skin irritation and photosensitivity may occur. This class of medications should not be used during pregnancy, especially tazarotene.
As of July 2016 the FDA approved adapalene as an over-the-counter medication for individuals 12 and older as of July 2016.
Acid preparations (e.g., salicylic acid, azelaic acid, glycolic acid) also decrease follicle hyperkeratinization. Salicylic acid is the most commonly used acid and is available over-the-counter. Azelaic acid may be effective for acne-induced hyperpigmentation.
Benzoyl peroxide is an effective topical treatment that has antibacterial and comedolytic properties. In patients with inflammatory lesions, it may be used in combination with a topical antibiotic, such as clindamycin, or a topical retinoid.
Extraction of comedones may also be performed by a trained clinician.
Inflammatory acne is often treated with multiple topical therapies, which are more effective than either agent alone. Benzoyl peroxide, topical antibiotics (e.g., erythromycin, dapsone, clindamycin), retinoids, and acids are commonly used.
Oral antibiotics (tetracyclines) are indicated for moderate inflammatory, pustular acne. Treatment should be brief as possible. An optimal goal is 3-4 months or when there is no response or further improvement in lesions. In menstruating females, specific formulations of hormonal contraceptives may also improve acne lesions.
Severe acne can be treated with intensive topical and oral treatment.
Oral antibiotics are indicated for an extended period of time. Resistance may occur with prolonged therapy.
Isotretinoin is usually reserved for the most severe cases of nodulocystic acne, or acne that is refractory to combination treatment. It is extremely effective, but is expensive and has many potential adverse effects, including teratogenicity. Close follow-up is necessary for laboratory work, including pregnancy tests, liver function tests, lipid panels, and complete blood counts. Treatment usually lasts 6 months and may result in permanent remission.
Systemic corticosteroids should be added if the acne worsens with initiation of isotretinoin. Intralesional steroid injection is also very effective for larger inflammatory lesions.
Many patients benefit from some level of maintenance therapy after their initial treatment phase. The best maintenance therapy is still being evaluated but topical retinoids are good option.
Several nutritional factors have been shown to be associated with the risk of acne:
Excess body weight. Acne is more common in overweight individuals, due in part to insulin resistance.
Western diets. Indigenous populations that eat largely plant-based diets composed mainly of unprocessed or minimally processed foods high in carbohydrate and fiber (e.g., tubers, fruit, vegetables, peanuts, corn, and rice), and emphasizing unsaturated rather than saturated fats, are largely free of acne. In contrast, acne is common in countries following Westernized diets. -, , As immigrants to Western countries become acculturated to a Western diet, their previously low incidence of acne rises to the levels found in Western societies.
Dairy products. The Nurses’ Health Study II, including more than 47,000 participants, showed that those who reported having consumed the most whole milk during adolescence were about 12 percent more likely to have had severe acne. Those who had consumed the most skim milk were 44 percent more likely to have had severe acne. That is, skim milk appeared to present greater risk than full-fat milk.
Because that study relied on participants’ memories or their diets as adolescents, the same researchers conducted a prospective study, including 4,273 boys and 6,094 girls, aged 9 to 15 years, followed over a three-year period. Study results mirrored the earlier findings. Milk consumption was associated with acne, and skim milk was implicated at least as much as whole milk. , Skim is much lower in fat, but higher in both protein and lactose.
Glycemic Load. Clinical trials have suggested that foods with a high glycemic index may contribute to acne, and low-glycemic-index foods may be helpful in treatment.  Examples of high-glycemic-index foods include sugar, wheat breads, white potatoes, and many cold cereals. Low-glycemic-index foods include most fruits, green vegetables, some grains (e.g., oats and barley), pasta, sweet potatoes, nuts, and beans and other legumes. Individually these foods are not necessarily problematic; rather, the problem likely results from the overall glycemic load of the diet, which may act by causing abnormalities in lipids and insulin, described below.
Inadequate zinc. Low serum zinc levels are associated with greater severity of acne lesions. Zinc supplements have been used to effectively treat inflammatory acne and are reportedly more effective for severe than mild to moderate acne, although gastrointestinal distress occurs more often with high doses.
The biological mechanisms linking Western diets, particularly dairy products, to acne appear to relate to hormonal effects. The effects of a surge in hormones (androgens) during puberty apparently combine with the ability of a Western diet to raise levels of IGF-1 and insulin, thereby making androgens and androgen precursors more available.
Androgens are synthesized from cholesterol derived from the bloodstream, and studies have revealed relationships between serum cholesterol concentrations and acne risk. Researchers have also observed worsening of acne with increasing intake of saturated fat (found in dairy products and meat) and trans fatty acids (commonly found in snack foods and fried foods).
The amino acid leucine and other branched-chain amino acids (which are present to a greater degree in animal than plant protein) increase IGF-1, act as precursors of sebaceous lipids, stimulate keratinocyte proliferation, and promote inflammatory activity of keratinocytes,  as well as increasing the availability of androgens and stimulating sebocyte production.  Dairy protein also contains steroids and steroid precursors that may stimulate sebum production and induce hyperkeratinization of the pilosebaceous unit. 
Plant-based diets may improve acne through an ability to lower IGF-I and increase IGF-binding proteins, in addition to an anti-inflammatory effect. 
Diets that represent a high glycemic load may cause hyperinsulinemia when regularly consumed. This results in an increase in IGF-1, a decrease in IGF binding proteins (IGFBPs), and a subsequent increase in keratinocyte proliferation and free androgens. Androgens in turn increase IGF-1, thereby causing a vicious circle. 
Low-fat, high-fiber, nondairy diet may be tried on a prospective basis.
Acne is a distressing condition. Some evidence suggests that it may be in some measure preventable by dietary changes. In particular, dairy intake has been associated with the development and worsening of acne in women and could also play a role in men. Likewise, a high-fat diet may be implicated in acne formation. In addition, the use of oil-based cosmetics, excessive scrubbing or rubbing, and occlusive clothing (tight bras, turtlenecks) may worsen the acne lesions. Family members can assist patients with acne by helping them stay on a healthful diet. Moreover, following such a diet themselves may reduce family members’ own risk for diet-related conditions.