Acne is a skin condition that develops when hair follicles become clogged with oil and dead skin cells, leading to bumps, pimples, and sometimes deeper, painful lesions. It affects roughly 10% of adolescents and young adults worldwide, with the highest rates among teenagers aged 15 to 19. While often thought of as a teenage problem, acne can persist well into adulthood and is about 25% more common in young women than in young men.
How Acne Forms
Your skin is covered in tiny hair follicles, each connected to an oil-producing gland. These glands make sebum, a waxy substance that normally keeps skin moisturized. Acne starts when four things go wrong at once: the glands produce too much oil, dead skin cells build up and block the follicle opening, bacteria multiply inside the clogged pore, and the immune system triggers inflammation in response.
The process often begins quietly. A follicle gets plugged before any visible blemish appears. Once trapped inside, a type of bacteria called Cutibacterium acnes (which normally lives harmlessly on skin) starts to thrive. Recent research has shown that acne isn’t simply about having too much of this bacterium. It’s about losing the natural diversity of bacterial strains on the skin. When one particularly aggressive strain dominates, it activates a specific inflammatory pathway that makes the surrounding skin red, swollen, and sore.
Why Hormones Play a Central Role
Hormones are the primary driver behind excess oil production. Androgens, a group of hormones that includes testosterone, directly stimulate oil glands to grow larger and produce more sebum. People with acne convert testosterone into a more potent form within the skin itself at higher rates than people without acne. This is why breakouts so often coincide with puberty, menstrual cycles, and other hormonal shifts. The oil glands contain all the enzymes needed to make this conversion happen locally, which means acne is partly a hormonal event happening right at the skin’s surface.
Types of Acne Lesions
Not all acne looks the same. The type of lesion depends on how deep the blockage sits and how much inflammation is involved.
- Whiteheads: Plugged follicles that stay sealed beneath the skin, appearing as small white bumps.
- Blackheads: Plugged follicles that open at the surface. The dark color comes from air reacting with the oil inside, not from dirt.
- Papules: Small, pink, inflamed bumps that are often tender to the touch.
- Pustules: Similar to papules but topped with a visible white or yellow pocket of pus, sometimes with a red base.
- Nodules: Large, solid, painful lumps lodged deep within the skin.
- Cystic acne: Deep, pus-filled lesions that are painful and carry the highest risk of scarring.
Whiteheads and blackheads are non-inflammatory, meaning there’s a blockage but the immune system hasn’t kicked in yet. Everything from papules onward involves inflammation, and the deeper the inflammation reaches, the more likely it is to leave a mark.
Genetics and Family History
If your parents had acne, your chances of developing it go up significantly. Twin studies estimate that up to 85% of the variation in who gets acne and how severe it becomes is explained by genetics. That doesn’t mean acne is inevitable if it runs in your family, but it does mean some people are biologically primed for oilier skin, stickier dead skin cells, or a stronger inflammatory response to clogged pores. Family history correlates not just with whether you get acne, but with how bad it gets.
How Diet Affects Breakouts
The connection between food and acne has been debated for decades, but the evidence has become clearer. High-glycemic foods, those that spike blood sugar quickly like white bread, sugary drinks, and sweets, have a modest but real effect on acne. In one study, people who followed a low-glycemic diet for several weeks saw their total lesion count drop by about 22, compared to roughly 11 in a control group. Inflammatory lesions specifically dropped by about 16 versus 6.
Dairy also appears to play a role, at least in Western diets. Drinking more than three servings of milk per week has been linked to a roughly 78% higher odds of moderate-to-severe acne. Skim milk shows a stronger association than whole milk, which may relate to the processing or the hormonal content of milk rather than its fat. That said, the evidence on dairy is entirely from observational studies, so it’s harder to be certain about cause and effect. The association appears weaker or absent in populations that don’t follow a Western-style diet.
The mechanism likely involves insulin. High-glycemic foods and dairy both raise insulin levels, which in turn can boost androgen activity and oil production in the skin.
How Common Treatments Work
Most acne treatment targets one or more of the four underlying factors: oil, dead skin buildup, bacteria, and inflammation.
Benzoyl peroxide, available over the counter in washes and creams, works by killing acne-causing bacteria directly. It releases oxygen inside the pore, which C. acnes can’t survive. It also has a mild effect on breaking down the dead skin cells that block follicles and slightly reduces oil production. Unlike antibiotics, bacteria don’t develop resistance to benzoyl peroxide, which makes it a reliable long-term option.
Salicylic acid takes a different approach. It’s oil-soluble, so it can penetrate into clogged pores and dissolve the buildup of dead skin cells from the inside. It’s particularly useful for blackheads and whiteheads.
Retinoids, which are derivatives of vitamin A available in both over-the-counter and prescription strengths, speed up skin cell turnover. This prevents dead cells from accumulating and plugging follicles in the first place. They’re often considered the foundation of acne treatment because they address the clogging that starts the whole process. Retinoids can cause dryness and irritation when you first start using them, which typically improves over several weeks.
For hormonal acne, particularly in women who break out along the jawline and chin in sync with their menstrual cycle, treatments that reduce androgen activity can be effective by cutting oil production at its hormonal source.
Scarring and Skin Changes
Acne can leave behind two distinct problems: scars and pigment changes. They’re often lumped together, but they’re fundamentally different. Pigment changes are flat marks, either dark (in deeper skin tones) or reddish (in lighter skin tones), that appear after a blemish heals. These are color changes, not structural damage to the skin, and they typically fade over months.
True scars involve changes to the skin’s structure. About 80 to 90% of acne scars are atrophic, meaning the skin lost collagen during healing and left behind a depression. These come in three shapes: ice pick scars (narrow and deep, making up 60 to 70% of atrophic scars), boxcar scars (round depressions with sharp edges), and rolling scars (broad, shallow dips caused by the skin being tethered to deeper tissue). A smaller number of people develop hypertrophic scars, where the body produces too much collagen during healing, creating raised, firm, pink bumps.
The risk of scarring increases with the severity and duration of inflammation. Nodular and cystic acne are far more likely to scar than surface-level breakouts, and picking or squeezing inflamed lesions makes scarring worse. Early treatment of inflammatory acne is one of the most effective ways to prevent permanent marks.