What actually causes acne?

Acne develops when four processes converge in a hair follicle: excess sebum production driven by androgen hormones, abnormal shedding of skin cells that plugs the follicle opening, colonization by Cutibacterium acnes bacteria, and the inflammatory response that produces redness and swelling. That is why acne peaks in adolescence when androgen levels rise, why it clusters in families, and why it appears on the face, chest, and back where sebaceous glands are densest. Adult acne is common, particularly in women, and is frequently hormonally driven.

The dietary folklore is mostly wrong. Chocolate and fried food as causes are not supported by good evidence. What has modest support is that high-glycemic-load diets, meaning foods that spike blood sugar rapidly, are associated with more acne, plausibly through insulin and insulin-like growth factor pathways, and that skim milk specifically shows an association in observational studies while whole milk and cheese generally do not. These effects are real but modest, and treating diet as the primary lever misallocates effort.

Effective treatment is well established and escalates by severity. Over the counter, benzoyl peroxide targets bacteria and salicylic acid helps unclog follicles, and adapalene, a retinoid, is now available without prescription and addresses the abnormal shedding at the root of the process. Prescription options include topical retinoids and antibiotics, oral antibiotics for a limited course, hormonal treatments including combined oral contraceptives and spironolactone, and isotretinoin for severe or scarring acne, which is highly effective and requires monitoring and strict pregnancy prevention. Two practical cautions: over-washing and harsh scrubbing worsen acne by damaging the barrier, and picking causes scarring and post-inflammatory pigmentation that outlast the original lesion by months or years.