Acne Cut

Acne Basics

What causes acne? How a spot forms, step by step

Drafted with AI assistance · not reviewed by a dermatologist · Published August 2, 2026 · Last updated October 5, 2026

Short answer

Acne forms when oil and dead skin cells plug a hair follicle, bacteria that normally live on the skin grow in the plug, and the area becomes inflamed [1][2]. Hormones called androgens drive the extra oil, which is why acne so often starts at puberty [1].

How settled

Well established

What we read

  1. National Institute of Arthritis and Musculoskeletal and Skin Diseases, Acne: overview, symptoms and causes
  2. Williams HC, Lancet 2012
  3. Tan JK, Br J Dermatol 2015
  4. NHS, Acne
  5. Scholz CFP, Int J Syst Evol Microbiol 2016
  6. National Institute of Arthritis and Musculoskeletal and Skin Diseases, Acne: diagnosis, treatment and steps to take
  7. Reynolds RV, J Am Acad Dermatol 2024
A small white jar, an amber dropper bottle and folded towels on a wet tiled counter
A jar, an amber bottle and towels. AI-generated illustration.

The parts involved

Two stacked cotton pads and a glass dropper resting on a small white dish
Cotton pads and a glass dropper. AI-generated illustration.

Each hair follicle in the skin has an oil gland, the sebaceous gland, attached to it. The gland makes sebum, an oil that helps keep the skin from drying out, and the sebum normally flows up the follicle and out onto the skin through the pore [1].

The follicle is lined with skin cells called keratinocytes. In healthy skin they shed and rise to the surface as new ones form [1]. So oil production is not a fault in itself; it is part of how skin protects itself. Acne begins when that flow gets blocked.

Diagram in four stages: a normal follicle with its oil gland; the gland enlarged and the opening plugged; bacteria multiplying below the plug; the follicle swollen, red and inflamed.
How a spot forms. Diagram drawn for Acne Cut from the descriptions in sources 1 and 2.

Step one: more oil

Androgens, often called male hormones although everyone has them, rise in both boys and girls at puberty. They make the oil glands larger and more productive [1]. A review in The Lancet describes acne as starting with androgen-driven increases in sebum, together with changes in how the follicle lining sheds [2].

Step two: the plug

In acne, hair, sebum and keratinocytes stick together inside the pore. The cells stop shedding properly and the sebum can no longer reach the surface [1]. Doctors call these plugged follicles comedones [1].

A plug that stays under the surface shows as a whitehead. A plug that reaches the surface and opens shows as a blackhead, and its dark color comes from air discoloring the sebum, not from dirt [1].

Step three: bacteria

The mix of oil and cells inside a blocked follicle lets bacteria that normally live on the skin grow [1]. The main one has long been known as Propionibacterium acnes [2]; in 2016 microbiologists moved it to a new genus, and it is now called Cutibacterium acnes [5].

Step four: inflammation

The bacteria and the trapped material set off inflammation: swelling, redness, heat and pain. When the wall of the plugged follicle breaks down, it spills bacteria, skin cells and sebum into the skin around it, creating the inflamed spots most people picture as acne [1].

Those spots come in grades. Papules are small, tender pink bumps; pustules are papules topped with white or yellow pus; nodules are large, painful, solid lumps deep in the skin; and severe nodular acne, sometimes called cystic acne, forms deep, painful, pus-filled lesions [1].

What tips some people into acne

Hormones are the main lever. Besides puberty, NIAMS names hormone changes in pregnancy [1], and the NHS lists periods and menopause too [4]. Family history plays a part: you may be more likely to get acne if your parents had it [1], and the Lancet review also notes that family history may be important [2].

Some medicines can cause acne, including ones that contain hormones, corticosteroids and lithium [1], and skin or hair products that leave the skin greasy can contribute [4]. Other things do not cause acne but can make it worse: stress, pressure from helmets, tight clothes or backpacks, pollution and humidity, picking or squeezing spots, and scrubbing too hard [1].

Three things that do not cause it

Dirt does not cause acne. The dark tip of a blackhead is sebum discolored by air, not grime [1], and the plug forms inside the follicle, below anything a face wash can reach [1]. Scrubbing harder does not help either: NIAMS lists scrubbing the skin too hard among the things that make acne worse, and advises cleaning gently with a mild cleanser and lukewarm water [1][6].

Food is not a proven cause. Diet has been implicated in acne but not proven [2], and NIAMS says some studies suggest certain foods may make acne worse while research continues [1].

And acne is not a sign that someone is doing something wrong. It is driven mostly by hormones and by how an individual's follicles behave, and it affects people of every background and age [1][3].

How common it is, and what is still unknown

Acne is one of the most common conditions in the world. A 2015 review estimated it affects 9.4 percent of the world's population, most often teenagers after puberty [3].

Researchers agree on the four steps, but not on exactly what sets them off in one person and not another. The Lancet review says exactly what triggers acne, and how treatment affects its course, remain unclear, and that diet has been implicated but not proven [2].

Why the steps matter for treatment

Each step is a target. Acne medicines work by tackling the clumping of cells in the follicle, high oil levels, bacteria or inflammation [6]. Benzoyl peroxide kills bacteria and may reduce oil, retinoids help treat spots and reduce inflammation, and salicylic acid helps break down blackheads and whiteheads [6].

That is also why treatments are often combined. The American Academy of Dermatology's 2024 guideline makes strong recommendations for benzoyl peroxide and topical retinoids, and recommends combining topical treatments that work in different ways [7].

The four evidence labels used on Acne Cut, with “Well established” highlighted for this article.
This article's label: Well established. The labels describe the research, not how likely something is to work for you.

Sources

  1. National Institute of Arthritis and Musculoskeletal and Skin Diseases. Acne: overview, symptoms and causes. NIH, last reviewed July 2023; accessed October 5, 2026. Read it
  2. Williams HC, Dellavalle RP, Garner S. Acne vulgaris. Lancet. 2012;379(9813):361–372. PubMed 21880356
  3. Tan JK, Bhate K. A global perspective on the epidemiology of acne. Br J Dermatol. 2015;172 Suppl 1:3–12. PubMed 25597339
  4. NHS. Acne. Page last reviewed September 22, 2026; accessed October 5, 2026. Read it
  5. Scholz CFP, Kilian M. The natural history of cutaneous propionibacteria, and reclassification of selected species within the genus Propionibacterium to the proposed novel genera Acidipropionibacterium gen. nov., Cutibacterium gen. nov. and Pseudopropionibacterium gen. nov. Int J Syst Evol Microbiol. 2016;66(11):4422–4432. PubMed 27488827
  6. National Institute of Arthritis and Musculoskeletal and Skin Diseases. Acne: diagnosis, treatment and steps to take. NIH, last reviewed July 2023; accessed October 5, 2026. Read it
  7. Reynolds RV, Yeung H, Cheng CE, et al. Guidelines of care for the management of acne vulgaris. J Am Acad Dermatol. 2024;90(5):1006.e1–1006.e30. PubMed 38300170

Read next