Acne develops when hair follicles become clogged with oil and dead skin cells, then inflamed. But the reasons that process kicks off in the first place vary widely, from hormones and genetics to diet, stress, medications, and the products you put on your skin. Most people with acne have several of these factors working together at once.
Hormones and Oil Production
Hormones called androgens are the single biggest driver of acne. Androgens bind to receptors inside the oil glands (sebaceous glands) and tell them to grow larger, multiply their cells, and produce more sebum, the waxy oil that keeps skin moisturized. During puberty, androgen levels surge, which is why acne so often appears in the teenage years. But hormonal shifts during menstrual cycles, pregnancy, polycystic ovary syndrome, and perimenopause can trigger breakouts well into adulthood.
The oil glands on your face and scalp are especially responsive because they contain high concentrations of an enzyme that converts testosterone into a more potent form. This is why acne clusters on the face, forehead, chest, and upper back rather than, say, on your shins. More sebum means a stickier environment inside the pore, which sets the stage for everything else that follows.
Clogged Pores and Dead Skin Cells
Even with extra oil, acne wouldn’t form if the pore stayed open. The problem is a process called follicular hyperkeratosis: the cells lining the inside of the hair follicle shed too fast and clump together instead of clearing out normally. These sticky cells mix with sebum to form a plug. That plug is a microcomedone, the invisible precursor to every whitehead, blackhead, and inflamed pimple. When the plug stays beneath the surface, you get a closed comedone (whitehead). When it reaches the surface and oxidizes, it darkens into a blackhead. Increased sebum production and this abnormal cell shedding together create the conditions for nearly all acne lesions.
Bacteria and Inflammation
A bacterium called Cutibacterium acnes (C. acnes) lives naturally on everyone’s skin. It’s not inherently harmful. But when a pore gets plugged, the low-oxygen, oil-rich environment inside becomes an ideal breeding ground, and the bacterial population explodes.
As C. acnes multiplies, it breaks down sebum into irritating fatty acids using enzymes called lipases. It also produces compounds called porphyrins that damage surrounding cells and trigger inflammation. The immune system responds aggressively: white blood cells flood the area, the follicle wall swells, and the result is the redness, tenderness, and pus of an inflamed pimple. In more severe cases, the follicle wall ruptures beneath the skin, spreading inflammation deeper and producing the painful nodules or cysts associated with severe acne.
C. acnes also produces enzymes that degrade hyaluronic acid in the skin’s outer layers, which helps the bacteria spread and fuels further inflammation. Different strains of the bacterium vary in how aggressively they provoke the immune system, which partly explains why two people with similar skin oiliness can have very different acne severity.
Genetics and Family History
If one or both of your parents had acne, your risk is substantially higher. A study comparing families of acne patients to families of unaffected individuals found that having a first-degree relative with adult acne nearly quadrupled the odds of developing it yourself (odds ratio of 3.93). Genetics appear to influence how your follicles behave over time. In people without a genetic predisposition, acne-prone follicles gradually become resistant to breakouts in early adulthood. In those with a family history, that transition often fails, which is why some people deal with acne well past their twenties.
Diet: Sugar and Dairy
Two dietary patterns show the strongest links to acne: high-glycemic foods and dairy.
Foods that spike blood sugar quickly (white bread, sugary drinks, white rice, candy) cause a cascade of hormonal signals. Your body releases more insulin, which in turn raises levels of a growth hormone called IGF-1. IGF-1 stimulates oil production and skin cell growth through many of the same pathways as androgens. A randomized controlled trial found that switching to a low-glycemic diet for just two weeks significantly decreased IGF-1 levels in adults with moderate to severe acne.
Dairy is the other consistent finding. A meta-analysis of over 78,000 children, adolescents, and young adults found that any dairy consumption was associated with higher odds of acne. Interestingly, skim and low-fat milk showed a stronger association (32% increased odds) than whole milk (22% increased odds). The likely explanation is that people who choose low-fat milk tend to drink more of it, increasing overall exposure to the hormones and bioactive molecules naturally present in milk. The exact mechanism is still being refined, but milk contains its own IGF-1 and other growth factors that may amplify the same hormonal pathways involved in breakouts.
Stress
The connection between stress and acne is more than anecdotal. Your skin has its own stress-response system that mirrors what happens in the brain. When you’re under psychological stress, oil glands ramp up production of corticotropin-releasing hormone (CRH), the same signaling molecule your brain uses to kick off the stress response. Research comparing acne-affected skin to clear skin found that CRH and its receptors were far more abundant in acne-involved sebaceous glands. This local stress response activates immune and inflammatory pathways in the skin itself, which can trigger new breakouts or worsen existing ones. It’s a feedback loop: stress worsens acne, visible acne increases stress, and the cycle continues.
Medications That Trigger Breakouts
A number of medications can cause acne or acne-like eruptions as a side effect. The most common culprits include:
- Corticosteroids: Both oral and topical forms can trigger breakouts, sometimes called “steroid acne,” typically appearing as uniform small bumps on the chest and back.
- Hormonal medications: Anabolic steroids used in bodybuilding are a well-known cause. Some hormonal contraceptives can also worsen acne, though others improve it.
- Anticonvulsants: Certain seizure medications, particularly phenytoin, are associated with acne-like eruptions.
- Lithium and some antipsychotics: Psychiatric medications including lithium and olanzapine can provoke breakouts.
- B12 supplements: High-dose vitamin B12 supplementation, especially combined with other B vitamins, has been linked to acneiform outbreaks in some cases.
- Cancer therapies: Drugs that target epidermal growth factor receptors frequently cause acne-like rashes as a side effect.
Drug-induced acne often looks slightly different from typical acne. It tends to appear suddenly, with lesions that are more uniform in size and may lack the blackheads and whiteheads seen in regular acne. If you notice breakouts starting shortly after beginning a new medication, that timing is worth noting.
Skincare and Cosmetic Products
Certain ingredients in makeup, moisturizers, sunscreens, and hair products can physically block pores and trigger a type of acne sometimes called acne cosmetica. Common offenders include lanolin derivatives (acetylated lanolin alcohol, acetylated wool fat), certain seaweed extracts, and heavy emollients. Despite marketing claims that a product’s overall formulation makes individual comedogenic ingredients safe, the pore-clogging properties of these ingredients remain the same regardless of how they’re blended.
Hair products deserve special attention. Oils, pomades, and heavy conditioners that contact the forehead, temples, or jawline are a frequent overlooked cause of breakouts in those areas. If your acne clusters along your hairline, your styling products may be the source.
Other Environmental and Physical Triggers
Friction and pressure on the skin, sometimes called acne mechanica, can provoke breakouts. Helmet straps, tight collars, backpack straps, and even the habit of resting your chin on your hand can create the combination of heat, pressure, and occlusion that pushes debris deeper into pores. Athletes who wear protective gear are particularly susceptible.
Humidity and sweating don’t directly cause acne, but sweat that sits on the skin mixes with oil and dead cells and accelerates pore blockage, especially under tight or non-breathable clothing. Occupational exposure to industrial chemicals, particularly halogenated hydrocarbons found in certain insecticides, herbicides, and electrical insulators, can cause a severe form of acne called chloracne, though this is rare outside of industrial settings.
Phone screens pressed against the cheek, pillowcases washed infrequently, and touching your face throughout the day all transfer bacteria and oil to the skin. None of these are root causes on their own, but in someone already prone to breakouts, they can tip the balance from a clogged pore staying quiet to one that becomes inflamed.