Understanding Acne:
What Causes It, and Which Type Do You Have?
Acne vulgaris is a chronic inflammatory condition of the pilosebaceous unit (the hair follicle and its attached oil gland) — not a hygiene problem, and not something anyone brings on themselves. It affects up to 85% of adolescents and remains common well into adulthood, in both men & women. Understanding what’s actually happening beneath the skin is the first step toward treating it effectively, and is the reason why every patient benefits from a thorough evaluation before recommending a treatment plan..
What Causes Acne?
(The Pathogenesis)
Acne develops through four interconnected processes happening inside the follicle. No single one of these “causes” acne on its own — it’s the combination, and the order in which they occur can vary from person to person.
-
Sebaceous glands attached to hair follicles produce sebum (oil) in response to androgen hormones. During puberty — and during hormonal shifts at other life stages — androgen activity rises, and sebaceous glands respond by producing more sebum than the skin needs. This oil-rich environment sets the stage for everything that follows.
-
Normally, dead skin cells (keratinocytes) lining the follicle shed in an orderly way. In acne-prone skin, these cells shed abnormally and clump together instead of releasing cleanly. Combined with excess sebum, this forms a plug — the microcomedone — that blocks the follicle opening. Every visible acne lesion can start as a microcomedone.
-
C. acnes (formerly classified as Propionibacterium acnes) is a bacterium that normally lives on everyone’s skin. Inside a blocked, oil-rich follicle, it finds an ideal low-oxygen environment and multiplies. It isn’t an “infection” in the sense of a foreign invader — it’s a normal resident bacterium overgrowing because conditions have changed in its favor.
-
C. acnes triggers the skin’s innate immune response, releasing inflammatory chemical signals that recruit immune cells to the area. This is what turns a plain clogged pore into a red, swollen, sometimes painful lesion. Newer research suggests inflammation may actually begin earlier in this process than once thought — potentially before the plug is even visible — rather than being purely a late-stage reaction to bacteria.
What Makes Acne Worse?
Several factors don’t directly cause acne but can trigger flares or increase severity:
Genetics — a family history of acne is one of the strongest predictors of developing it
Hormonal fluctuations — puberty, the menstrual cycle, pregnancy, and conditions like polycystic ovary syndrome (PCOS)
Certain medications — including some oral birth control or devices, corticosteroids, anticonvulsants, and lithium
Comedogenic products — heavy, pore-clogging cosmetics or hair products
Friction and pressure — masks, helmets, chinstraps, and phone use against the cheek (sometimes called acne mechanica)
Diet — an evolving area of research; see our Acne Treatment page for what the evidence actually shows
Stress — associated with flares, likely through its effects on cortisol levels, hormones, and inflammation
Types of Acne
Dermatologists classify acne two ways: by the type of lesion present, and by overall severity. Most patients have a mix of lesion types at once.
By Lesion Type:
Non-inflammatory lesions (comedones)
Closed comedones (whiteheads) — a plugged follicle covered by a thin layer of skin, appearing as a small, flesh-colored or whitish bump
Open comedones (blackheads) — a plugged follicle open to the air; the dark color comes from oxidized melanin and debris, not dirt
Inflammatory lesions
Papules — small, firm, red, tender bumps without visible pus
Pustules — similar to papules but with a visible white or yellow pus-filled center
Nodules — larger, firm, deep lesions that form when inflammation extends deeper into the skin; often painful
Cysts — deep, pus-filled, painful lesions; nodules and cysts together define “nodulocystic” acne, the most severe common form
BY SEVERITY:
Dermatologists estimate severity using lesion counts and validated tools such as the Investigator’s Global Assessment (IGA) scale:
Mild — Mostly comedones, with few inflammatory lesions (papules/pustules only)
Moderate — More numerous comedones and inflammatory lesions; may include an occasional small nodule; often affects more than half the face
Severe — Widespread comedones and inflammatory lesions with multiple nodules or cysts; higher risk of scarring
Severity — not just lesion count — guides treatment choice, which is why we grade every new patient at their first visit rather than relying on a self-assessment.
Special Presentations of acne:
Hormonal (adult) acne — inflammatory lesions concentrated along the jawline and chin, more common in adult women, often cycling with the menstrual cycle
Acne mechanica — breakouts localized to areas of friction or pressure (under straps, masks, helmets, tight collars)
Acne conglobata — a rare, severe, interconnected form with deep nodules and abscesses, more common in men
Acne fulminans — a rare, sudden, severe form that can involve fever and joint pain; requires urgent dermatologic care
Neonatal and infantile acne — breakouts in newborns and infants, usually self-limited and different in cause from adolescent/adult acne
Drug-induced acne — breakouts triggered or worsened by a specific medications or vitamins/supplements
When to See a Dermatologist?
Over-the-counter (OTC) care is reasonable for mild, comedonal acne (see our Acne Treatment page). It’s worth booking a consultation if you notice any of the following:
Nodules or cysts, or acne that’s painful rather than just cosmetically bothersome
Acne that hasn’t improved after 2–3 months of consistent OTC treatment
Any scarring, or dark/red marks that persist after a lesion clears
Acne that’s affecting your confidence or daily life
Sudden, severe onset, especially with fever or joint pain
