Acne forms when oil glands in your skin produce too much sebum, dead skin cells clog your pores, bacteria multiply inside those clogged pores, and your immune system responds with inflammation. Those four steps, working together, explain nearly every pimple, blackhead, and cyst. But the reasons those steps get triggered in the first place range from your hormones and your genes to what you eat and how much stress you’re under.
How a Pimple Actually Forms
Every pore on your face and body is the opening of a tiny hair follicle. Lining each follicle are skin cells that normally shed and get pushed out to the surface. When those cells become sticky and cohesive instead of shedding normally, they form an invisible plug deep inside the pore called a microcomedone. You can’t see or feel it yet, but it’s the seed of every acne lesion.
Behind that plug, your sebaceous (oil) glands keep pumping out sebum with nowhere for it to go. The trapped oil creates an oxygen-poor environment that’s ideal for a bacterium called C. acnes, which lives on everyone’s skin but thrives when sealed inside a clogged pore. Certain strains of this bacterium have rigid cell wall structures that are especially good at provoking an immune response. Your body sends inflammatory signals to fight the bacteria, and the result is redness, swelling, and pus. That’s the progression from invisible clog to whitehead to angry, inflamed breakout.
Hormones Are the Biggest Driver
The oil glands in your skin are essentially hormone-responsive organs. Androgens, a group of hormones that includes testosterone, directly stimulate sebum production. Your oil glands even contain the enzymes needed to convert weaker androgens into more potent forms like dihydrotestosterone (DHT), which ramps up oil output even further. This is why acne typically begins at puberty: androgen levels surge, oil production spikes, and pores that never caused problems before start clogging.
In girls, acne onset correlates with rising levels of a specific androgen precursor called DHEAS, sometimes appearing even before other signs of puberty. Hormonal fluctuations also explain why many women break out around their periods, during pregnancy, or after stopping birth control. The oil glands respond to even small hormonal shifts, which is why acne can persist well beyond the teenage years. Roughly one in three women in their 30s still deals with acne to some degree.
Your Genes Set the Stage
If your parents had acne, your chances of developing it increase substantially. Twin studies estimate that about 81% of the variation in acne severity is attributable to genetics. A large study of over 1,200 first-degree relatives found that having a parent or sibling with adult acne nearly quadrupled the odds of having it yourself, compared to people with no family history.
What you inherit isn’t acne itself but the underlying traits that make it more likely: how much oil your skin produces, how readily your pore lining sheds (or doesn’t), and how aggressively your immune system reacts to clogged follicles. This genetic foundation explains why two people with similar diets, stress levels, and skincare routines can have completely different skin.
How Diet Affects Breakouts
The link between food and acne was dismissed for decades, but the evidence has shifted. High glycemic load foods, those that spike your blood sugar quickly (white bread, sugary drinks, processed snacks), have compelling evidence connecting them to worse acne. When blood sugar rises sharply, your body releases insulin and a related growth factor called IGF-1. Both increase androgen activity and oil production, feeding the cycle that leads to clogged pores.
Dairy has a weaker but consistent association with acne across multiple studies. Whether this comes from the natural hormones present in milk or from dairy’s ability to raise IGF-1 levels isn’t fully settled. The practical takeaway: cutting back on sugary, highly processed foods is more likely to help your skin than eliminating any single food group. Dairy may matter for some people but probably isn’t a major driver for most.
Stress and the Brain-Skin Connection
Your skin has its own version of the stress response system. When you’re under psychological stress, your brain activates the HPA axis, the same hormonal cascade that produces cortisol. Your skin cells, including the ones lining your oil glands, can detect and respond to these stress signals locally. Stress hormones like corticotropin-releasing hormone (CRH) and neuropeptides like Substance P directly increase oil production and trigger inflammation in the skin.
This brain-skin axis means that exam weeks, job pressure, or sleep deprivation don’t just make you feel worse. They chemically change how much oil your skin makes and how reactive your immune system is to clogged pores. It’s a real biological pathway, not just anecdotal.
Friction, Heat, and Pressure Breakouts
Not all acne is hormonal. Acne mechanica is a specific type of breakout triggered by friction, pressure, heat, and occlusion against the skin. It shows up as inflamed bumps and pustules in areas where something rubs repeatedly: under helmet chin straps, along bra straps, beneath heavy backpack straps, or where skin rubs against skin during physical activity.
This type became widely recognized during the pandemic when “maskne” affected people wearing face coverings for hours. Athletes are especially prone, particularly football players who develop breakouts under pads and helmets. Even skin-on-skin friction can cause it. One documented case involved blackheads forming on a woman’s inner thighs where her legs rubbed together in warm weather. Wearing a clean, absorbent cotton layer between your skin and any gear or tight clothing helps reduce the four contributing factors: occlusion, heat, friction, and pressure.
Why Acne Persists Into Adulthood
Acne isn’t just a teenage problem. Population studies show that about 31% of adult women still have acne, making it one of the most common skin conditions at any age. Adult acne tends to concentrate along the jawline and chin, driven largely by hormonal fluctuations rather than the widespread oiliness of adolescent skin. For many adults, the condition is persistent rather than something that clears and returns. It often runs in families, with adult-onset acne showing a particularly strong genetic component.
The combination of factors that causes acne, hormones, genetics, bacterial balance, immune reactivity, and environmental triggers, doesn’t neatly resolve when puberty ends. Each person’s mix of contributing factors is different, which is why acne responds to different treatments depending on what’s driving it. Someone whose acne is primarily hormonal may not improve much from topical treatments alone, while someone whose breakouts are triggered by diet or friction may see rapid improvement from lifestyle changes.