The main cause of acne is excess oil production driven by hormones, specifically androgens. But acne is rarely the result of a single factor. It develops when four processes collide: your skin produces too much oil, dead skin cells clog your pores, bacteria multiply inside those clogged pores, and your immune system triggers inflammation. Hormones are the engine behind most of this chain reaction, which is why acne hits hardest during puberty and remains common well into adulthood.
How Hormones Drive Oil Production
Androgens, a group of hormones that includes testosterone, are the primary trigger. During puberty, androgen levels surge in both boys and girls. These hormones signal the oil glands in your skin to grow larger and produce more sebum, the waxy, oily substance that normally keeps skin moisturized. The most potent androgen involved is dihydrotestosterone (DHT), which can push immature oil gland cells into a full lipid-producing state on its own, causing them to fill with triglycerides and other fats that eventually reach the skin’s surface.
This is why acne affects roughly 85% of people between ages 12 and 25. But hormonal fluctuations don’t stop after the teenage years. Up to 20% of women and 8% of men deal with acne as adults, often linked to menstrual cycles, pregnancy, polycystic ovary syndrome, or perimenopause. Any condition that raises androgen levels or makes oil glands more sensitive to them can restart the cycle.
The Clogged Pore Problem
Oil alone doesn’t cause a breakout. The trouble starts when dead skin cells inside a pore don’t shed the way they should. Normally, the lining of a hair follicle pushes old cells to the surface, where they flake away unnoticed. In acne-prone skin, this process goes wrong. The stem cells responsible for maintaining the pore lining shift toward producing thicker, stickier cells that clump together instead of shedding. Researchers call this the “comedo switch,” and it’s now understood to be a change in how specific stem cells in the follicle differentiate.
The result is a microcomedone, a tiny plug of dead cells and oil invisible to the naked eye. Every whitehead, blackhead, and inflamed pimple begins as one of these microscopic blockages. If the plug stays small and near the surface, you get a blackhead (the dark color comes from oxidation, not dirt). If it seals over entirely, oil and bacteria build up underneath, and you get a whitehead or, eventually, an inflamed lesion.
The Role of Bacteria
A bacterium called Cutibacterium acnes lives naturally on everyone’s skin. It feeds on sebum and thrives in the low-oxygen environment inside a clogged pore. In small numbers, it’s harmless. But when a pore is sealed and flooded with oil, C. acnes multiplies rapidly, and that’s when your immune system gets involved.
Live C. acnes releases RNA that activates a specific receptor on immune cells called TLR8. This triggers natural killer cells to release signaling molecules that ramp up inflammation. The redness, swelling, and tenderness of a pimple aren’t caused by the bacteria directly. They’re caused by your own immune system attacking the overgrowth. This is also why “popping” a pimple often makes it worse: rupturing the follicle wall spreads bacterial material into surrounding tissue and intensifies the immune response.
Genetics Set the Stage
If both your parents had acne, your chances of developing it are significantly higher. A twin study found that 81% of the variation in acne severity was attributable to genetic factors. That’s a remarkably strong genetic influence, on par with height. Your genes determine how large your oil glands are, how sensitive they are to androgens, how your immune system responds to C. acnes, and how readily your pore linings produce sticky cells. This explains why some people break out severely while others with the same diet and hygiene barely get a pimple.
Diet and Blood Sugar
Foods that spike your blood sugar can worsen acne through a specific hormonal pathway. When you eat high-glycemic foods like white bread, sugary drinks, or processed snacks, your body produces more insulin. Elevated insulin increases levels of a hormone called insulin-like growth factor 1 (IGF-1), which in turn stimulates oil glands and boosts androgen activity. In a randomized controlled trial, participants who switched to a low-glycemic diet saw a significant drop in IGF-1 levels in just two weeks.
Dairy has also been linked to acne, likely because milk naturally contains hormones and bioactive molecules that influence insulin and IGF-1. The connection is strongest with skim milk, though researchers are still working out exactly why fat content matters. These dietary effects don’t cause acne on their own, but in someone already predisposed, they can tip the balance.
Stress and the Skin’s Hormone System
Your skin has its own stress-response system, separate from the one in your brain. When you’re under psychological stress, skin cells release a hormone called corticotropin-releasing hormone (CRH), which kicks off a local cascade of stress hormones right in the skin itself. This leads to increased production of cortisol and inflammatory molecules like interleukin-6 directly at the site of your pores and oil glands.
The result is a double hit: stress hormones can stimulate oil production while simultaneously ramping up inflammation, making existing breakouts worse and new ones more likely. This explains the common experience of breaking out before exams, job interviews, or other high-pressure events. The flare isn’t imagined. It’s a measurable biological response.
Physical Friction and External Triggers
A specific type of acne called acne mechanica develops where skin is pressed, rubbed, or trapped under gear. Football players commonly get it along the chin from helmet straps. Backpack straps, tight sports bras, and headbands can all create the conditions for it: heat, sweat, friction, and occlusion. The combination traps oil and dead cells against the skin, fast-tracking the clogging process. Wearing a clean, absorbent cotton layer between skin and equipment helps reduce all four contributing factors.
Medications That Trigger Breakouts
Several medications can cause or worsen acne as a side effect. Corticosteroids are among the most common culprits, whether taken orally, inhaled, or applied to the skin in large amounts. Anabolic steroids cause acne in about 50% of users at high doses. Lithium, used to treat mood disorders, frequently triggers breakouts during long-term use. Certain cancer drugs that target a growth receptor on skin cells cause acne-like eruptions in roughly 85% of patients.
First-generation progestins found in some hormonal contraceptives can also worsen acne because they have testosterone-like effects. Even high doses of B12 supplements have been linked to flares. If your breakouts started or worsened after beginning a new medication, that connection is worth exploring with your prescriber.
Why It All Comes Back to Hormones
Each of these factors, whether it’s genetics, diet, stress, or friction, ultimately feeds into the same core process. Androgens drive oil production. Excess oil feeds bacteria. Abnormal skin cell shedding traps everything inside the pore. And your immune system inflames the whole area in response. The “main cause” is hormonal stimulation of the oil glands, but what makes acne so persistent and so individual is that multiple triggers converge on this pathway at once. Two people with the same hormone levels can have very different skin simply because of differences in genetics, diet, or stress load. Understanding which factors are most active for you is what makes targeted treatment possible.