Side effects include increased skin photosensitivity, dryness, redness and occasional peeling.[80] Sunscreen use is often advised during treatment, to prevent sunburn. Lower concentrations of benzoyl peroxide are just as effective as higher concentrations in treating acne but are associated with fewer side effects.[79][81] Unlike antibiotics, benzoyl peroxide does not appear to generate bacterial antibiotic resistance.[80]

C. acnes also provokes skin inflammation by altering the fatty composition of oily sebum.[45] Oxidation of the lipid squalene by C. acnes is of particular importance. Squalene oxidation activates NF-κB (a protein complex) and consequently increases IL-1α levels.[45] Additionally, squalene oxidation leads to increased activity of the 5-lipoxygenase enzyme responsible for conversion of arachidonic acid to leukotriene B4 (LTB4).[45] LTB4 promotes skin inflammation by acting on the peroxisome proliferator-activated receptor alpha (PPARα) protein.[45] PPARα increases activity of activator protein 1 (AP-1) and NF-κB, thereby leading to the recruitment of inflammatory T cells.[45] The inflammatory properties of C. acnes can be further explained by the bacterium's ability to convert sebum triglycerides to pro-inflammatory free fatty acids via secretion of the enzyme lipase.[45] These free fatty acids spur production of cathelicidin, HBD1, and HBD2, thus leading to further inflammation.[45]

Acne inversa (L. invertō, "upside down") and acne rosacea (rosa, "rose-colored" + -āceus, "forming") are not true forms of acne and respectively refer to the skin conditions hidradenitis suppurativa (HS) and rosacea.[26][27][28] Although HS shares certain common features with acne vulgaris, such as a tendency to clog skin follicles with skin cell debris, the condition otherwise lacks the defining features of acne and is therefore considered a distinct skin disorder.[26]
From the What to Expect editorial team and Heidi Murkoff, author of What to Expect the First Year. Health information on this site is based on peer-reviewed medical journals and highly respected health organizations and institutions including ACOG (American College of Obstetricians and Gynecologists), CDC (Centers for Disease Control and Prevention) and AAP (American Academy of Pediatrics), as well as the What to Expect books by Heidi Murkoff.

Pimples are raised red spots with a white center that develop when blocked hair follicles become inflamed or infected with bacteria. Blockages and inflammation that develop deep inside hair follicles produce cystlike lumps beneath the surface of your skin. Other pores in your skin, which are the openings of the sweat glands, aren't usually involved in acne.

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