Acne develops when four things happen inside your skin at roughly the same time: oil glands produce too much sebum, dead skin cells stick together and block the pore, bacteria multiply in that blocked pore, and your immune system responds with inflammation. Nearly 10% of people aged 10 to 24 worldwide have acne at any given time, and the rate is about 25% higher in young women than in young men.
How a Pimple Actually Forms
Every pore on your skin sits at the top of a tiny channel called a hair follicle, which contains an oil-producing gland. These glands constantly release sebum, a waxy substance that keeps your skin moisturized. Under normal conditions, sebum flows up through the follicle and spreads across the skin’s surface without any issues.
The trouble starts when dead skin cells inside the follicle don’t shed properly. Instead of sloughing off and washing away, they clump together and form a sticky plug at the opening of the pore. This microscopic blockage is called a microcomedone, and it’s the earliest stage of every acne lesion, from a small whitehead to a deep, painful cyst. Once that plug forms, sebum keeps being produced but has nowhere to go. It pools behind the blockage, creating a small, oxygen-poor pocket that becomes an ideal environment for bacteria already living on your skin.
The Role of Hormones
Hormones are the single biggest reason oil glands go into overdrive. Androgens, a group of hormones that includes testosterone, bind to receptors on the surface of sebaceous gland cells. Those receptors are especially concentrated in the glands on your face and scalp. When androgens dock onto these receptors, they trigger the gland cells to multiply and produce more fat, which translates directly into more sebum.
This is why acne so often appears during puberty. Rising androgen levels cause a dramatic increase in sebum output at exactly the age when most teenagers first develop breakouts. Your skin also contains an enzyme that converts testosterone into a more potent form, and sebaceous glands on the face produce this enzyme in abundance. So even modest increases in circulating testosterone get amplified locally in the skin.
Hormonal shifts explain adult acne, too. About 80% of women with adult acne report flare-ups before menstruation, and roughly 70% have noticeably oily skin. Adult female acne tends to cluster along the jawline, chin, and lower cheeks (the “U-zone”), while teenage acne is more common across the forehead, nose, and upper cheeks (the “T-zone”).
Why Bacteria Make It Worse
A species called Cutibacterium acnes lives on virtually everyone’s skin. It’s a normal part of the skin microbiome, and many strains are completely harmless. The problem is that not all strains behave the same way. Acne-associated strains produce higher levels of porphyrins, small molecules that interact with skin cells and trigger an aggressive immune response.
Here’s what happens at the cellular level: porphyrins from these acne-prone strains cause potassium to leak out of your skin cells. That potassium loss activates an internal alarm system called the inflammasome, which in turn releases a powerful inflammatory signal. This signal recruits immune cells to the area, producing the redness, swelling, and tenderness you recognize as an inflamed pimple. Strains found on healthy skin produce far fewer porphyrins and don’t trigger the same cascade, which is why two people can carry the same bacterial species with very different outcomes.
These bacteria also form a sticky film on the inside of the follicle wall, which increases the adhesion of dead skin cells and makes the initial pore blockage worse. So the bacteria don’t just cause inflammation after a clog forms; they actively contribute to the clogging itself.
Stress and Cortisol
Stress doesn’t cause acne on its own, but it reliably makes existing acne worse. When you’re under sustained stress, your body releases cortisol. Cortisol directly increases sebaceous gland activity, boosting sebum production in the same way androgens do. One study found a statistically significant correlation between cortisol levels and acne severity, on par with the correlation seen for testosterone and other androgens.
Stress also elevates prolactin and certain thyroid hormones. Your skin has receptors for both, and their activation further increases oil and sweat gland secretion. This layered hormonal response helps explain why breakouts often coincide with exam periods, work deadlines, or other prolonged stressful episodes.
Diet’s Connection to Breakouts
Foods that spike your blood sugar quickly, like white bread, sugary drinks, and processed snacks, raise insulin levels. Elevated insulin stimulates your body to produce more insulin-like growth factor-1 (IGF-1), a hormone that promotes skin cell growth and increases sebum production. A two-week randomized controlled trial found that switching to a low-glycemic diet reduced IGF-1 levels in people with moderate to severe acne.
This doesn’t mean a single candy bar causes a pimple. The connection is about patterns: consistently eating high-glycemic foods keeps IGF-1 elevated, which feeds into the same cycle of excess oil and abnormal skin cell turnover that drives acne from the inside.
Genetics and Family History
If your parents had acne, your chances of developing it are significantly higher. Twin studies using medical records from large registries have consistently found a substantial genetic influence on acne. Researchers assessed heritability through three different statistical methods, and all three pointed to the same conclusion: your genes play a major role in how your sebaceous glands behave, how your skin cells shed, and how aggressively your immune system responds to clogged pores.
This genetic component helps explain why some people follow every skincare recommendation and still break out, while others can be fairly careless with their routine and stay clear. Genetics set the baseline; everything else modulates it.
External Factors That Clog Pores
Some skincare and hair products contain ingredients that are inherently comedogenic, meaning they block pores regardless of how the product is formulated. Common culprits include certain lanolin derivatives, carrageenan (a thickener derived from seaweed), and some plant oils like carrot seed oil. Despite marketing claims that a product’s overall formulation neutralizes these ingredients, the comedogenic property of individual ingredients doesn’t change based on what else is in the bottle.
Friction is another overlooked trigger. Tight helmet straps, chin guards, headbands, or the habit of resting your chin in your hands can create localized breakouts by physically pressing oil and dead skin cells deeper into follicles. This form of acne, sometimes called acne mechanica, appears exactly where the pressure occurs and resolves when the source of friction is removed.
Why It Differs by Age and Sex
Teenage acne and adult acne share the same underlying mechanisms but express differently. Adolescent acne typically presents as a mix of blackheads, whiteheads, and inflamed papules spread across the forehead and cheeks, often extending to the chest and back. Truncal involvement is common in teenagers but rare in adults.
Adult acne, particularly in women, favors the lower face. In one study, 81% of adults with acne had cheek involvement, 67% had chin involvement, and 58% had breakouts along the jawline. Only about 2% had trunk involvement. The lesions tend to be deeper, forming small nodules and cysts rather than the surface-level whiteheads more typical of teenage skin. Hormonal fluctuations, chronic stress, resistant bacterial strains, and comedogenic cosmetics are all considered contributing factors in adult-onset cases.