Acne develops when hair follicles in the skin become clogged with oil and dead skin cells, then get inflamed. About 85% of people between ages 12 and 25 experience it, and it remains common well into adulthood, affecting up to 20% of women and 8% of men past age 25. While it looks like a surface problem, acne is driven by a chain reaction involving hormones, skin cell behavior, bacteria, and your immune system. Understanding each link in that chain explains why breakouts happen when they do, and why they vary so much from person to person.
The Four Steps Behind Every Breakout
Every acne lesion, from a tiny blackhead to a deep cyst, follows the same basic sequence. First, skin cells lining a hair follicle start multiplying faster than normal. Instead of shedding and clearing out of the pore, they pile up and form a plug. This process, called hyperkeratinization, is the earliest physical change. Biopsies of acne-prone skin show that even follicles that look perfectly clear already have elevated cell turnover compared to non-acne-prone skin, which means plugging is baked into the biology before a visible pimple ever forms.
Second, the sebaceous gland attached to the follicle produces excess oil (sebum). That oil gets trapped behind the plug, swelling the pore into what dermatologists call a microcomedone, the microscopic seed of every pimple. Third, bacteria that naturally live on your skin, particularly a species called Cutibacterium acnes, thrive in that oxygen-poor, oil-rich environment and multiply rapidly. Fourth, your immune system detects those bacteria and mounts an inflammatory response, sending immune cells and signaling molecules to the site. That inflammation is what turns a clogged pore into a red, swollen, sometimes painful lesion.
Hormones Are the Primary Trigger
Androgens, a group of hormones that includes testosterone, are the main reason sebaceous glands ramp up oil production. Both men and women produce androgens, and levels surge during puberty, which is why acne so often appears in the teenage years. Inside the skin, testosterone gets converted into a more potent form called dihydrotestosterone (DHT). When DHT binds to receptors on sebaceous gland cells, it triggers those cells to grow larger, produce more fat molecules, and essentially pump out more sebum. People born without functional androgen receptors do not produce sebum and do not develop acne, which is one of the clearest pieces of evidence that this hormonal pathway is essential.
This is also why acne patterns differ between men and women. Men tend to have more severe acne during their teens because of higher overall androgen levels. Women, on the other hand, experience breakouts tied to hormonal fluctuations throughout life. About 50% of women in their 20s and 25% of women in their 40s deal with hormonally driven acne. Menstrual cycles, pregnancy, menopause, and stopping birth control all shift hormone levels in ways that can increase oil production and trigger flares, often concentrated along the jawline and chin.
Inflammation Starts Earlier Than You’d Think
For decades, the standard explanation was that inflammation only showed up after a pore was already clogged and colonized by bacteria. Newer research has overturned that timeline. Biopsies of skin that looks completely normal in acne-prone individuals reveal elevated numbers of immune cells, including T cells and macrophages, clustered around follicles that haven’t yet formed a visible plug. Levels of the inflammatory signaling molecule interleukin-1 are also already elevated in these areas.
This means inflammation isn’t just a reaction to clogged pores. It’s part of the setup. Low-grade immune activation in the skin may actually contribute to the abnormal cell turnover that forms the plug in the first place. Once bacteria do proliferate inside a blocked pore, they amplify the immune response dramatically, triggering a cascade of inflammatory signals that cause redness, swelling, and tissue damage. Air pollution particles can worsen this cycle further by intensifying the inflammatory signals that skin cells produce when exposed to acne bacteria.
Genetics Explain Most of the Variation
If your parents had acne, your chances of developing it go up significantly. A large twin study found that 81% of the variation in acne severity was attributable to genetic factors, with only 19% linked to individual environmental differences. That’s a remarkably high heritability, comparable to height.
What gets inherited isn’t “acne” as a single trait but rather a collection of predispositions: how sensitive your sebaceous glands are to androgens, how quickly your follicular skin cells turn over, how aggressively your immune system responds to bacteria, and even the composition of your skin’s microbial community. This is why two teenagers with similar diets, stress levels, and skincare routines can have completely different skin. It also explains why acne runs so strongly in families and why some people never get more than the occasional pimple while others deal with deep cystic lesions.
How Diet Feeds Into the Process
Diet doesn’t cause acne on its own, but it can make existing acne worse through a specific hormonal pathway. Foods that spike blood sugar quickly, like white bread, sugary drinks, and processed snacks, cause a surge in insulin. Elevated insulin raises levels of a hormone called insulin-like growth factor 1 (IGF-1), which directly stimulates sebaceous glands to produce more oil and promotes the kind of rapid cell growth that clogs pores. A randomized controlled trial found that switching to a low glycemic diet for just two weeks measurably decreased IGF-1 levels in people with moderate to severe acne.
Dairy is the other dietary factor with consistent links to acne. Milk contains both casein and whey protein, which raise IGF-1 and insulin levels respectively. Dairy also naturally contains small amounts of androgens and other hormones that can contribute to sebaceous gland activity. A meta-analysis of observational studies confirmed the association between dairy intake and acne development, though the effect appears strongest with skim milk. The mechanism makes biological sense: dairy activates the same hormonal pathways that drive oil production and pore clogging. That said, the effect size varies from person to person, and cutting dairy doesn’t clear acne for everyone.
Why Acne Persists Into Adulthood
Many people expect acne to disappear after their teenage years, and for some it does. But adult acne is increasingly common, particularly in women. Hormonal fluctuations don’t end at puberty. The menstrual cycle creates a monthly rhythm of rising and falling hormones, and many women notice breakouts in the week before their period when progesterone rises and has mild androgenic effects. Life transitions like pregnancy, perimenopause, and stopping hormonal contraceptives can create prolonged shifts in hormone balance that sustain acne well into the 30s and 40s.
Stress compounds the problem. Chronic stress elevates cortisol, which can increase androgen activity in the skin. The same genetic predispositions that made someone acne-prone at 15, hyperresponsive sebaceous glands, sticky follicular cells, an overactive inflammatory response, don’t go away with age. They just get triggered by different things. For adults who never had teenage acne but develop it later, the underlying biology is the same; the hormonal trigger is simply arriving on a different timeline.
Putting It All Together
Acne isn’t caused by one thing. It’s the result of several biological systems intersecting: hormones that overstimulate oil glands, skin cells that multiply too fast and block pores, bacteria that flourish in that environment, and an immune system that responds with inflammation. Genetics set the baseline for how reactive each of those systems is, accounting for about four-fifths of your risk. Diet, hormonal fluctuations, and environmental factors act on top of that genetic foundation, dialing the severity up or down. This is why acne is so universal during puberty, when androgen levels surge in nearly everyone, and why it varies so much in severity and duration from one person to the next.