Acne is caused by four things happening inside your hair follicles at once: overproduction of oil, a buildup of dead skin cells that plugs the pore, overgrowth of specific bacteria, and inflammation. These four factors feed off each other in a cycle, and the reason they kick into gear varies from person to person, driven by hormones, genetics, diet, and stress.
The Four Processes Behind Every Breakout
Every pimple starts in a hair follicle. The cells lining the follicle normally shed and get pushed out, but in acne-prone skin, those cells multiply faster and stick together instead of sloughing off. This creates a tiny plug deep in the pore called a microcomedone, the invisible precursor to every whitehead, blackhead, and inflamed lesion.
At the same time, the oil glands attached to the follicle are producing excess sebum. That oil gets trapped behind the plug, creating an oxygen-poor environment where a specific bacterium thrives. The bacterium, called Cutibacterium acnes, lives on everyone’s skin, but certain strains are far more likely to trigger breakouts. Strains classified as phylotype IA1 and IA2 are consistently found in higher numbers on acne-prone skin, while healthier skin tends to host a more diverse mix of strains, particularly types IB and II. It’s not that the bacteria are foreign invaders. It’s that the wrong strains dominate when conditions shift in their favor.
Once those bacteria multiply inside a clogged, oil-rich pore, they trigger the immune system. Your skin cells and immune cells detect the bacteria and launch an inflammatory response, flooding the area with signaling molecules that cause redness, swelling, and eventually pus. This is why some clogged pores stay as painless blackheads while others become red, tender bumps. The degree of immune activation determines how severe the lesion becomes.
Why Hormones Are the Biggest Driver
Androgens, the group of hormones that includes testosterone, are the primary reason oil glands ramp up production during puberty and beyond. Testosterone itself isn’t the most potent trigger. Inside the oil gland, an enzyme converts testosterone into a much stronger form called dihydrotestosterone (DHT). Research has found that acne-affected skin produces 2 to 20 times more DHT than clear skin in the same areas of the body. DHT binds to receptors in the oil gland, enters the cell nucleus, and directly increases sebum output.
This is why acne peaks during puberty, when androgen levels surge. It also explains hormonal acne patterns in adults: breakouts that flare around menstrual cycles, during pregnancy, or with conditions like polycystic ovary syndrome all trace back to shifts in androgen activity. The oil glands are essentially following hormonal instructions, and some people’s glands are more sensitive to those signals than others.
Genetics Set Your Baseline Risk
A large twin study found that 81% of the variation in acne severity was attributable to genetic factors. If both your parents had significant acne, your odds of developing it are substantially higher than average. What you inherit isn’t acne itself but the underlying traits that make it more likely: how sensitive your oil glands are to hormones, how quickly your skin cells turn over inside pores, and how aggressively your immune system responds to bacterial colonization. These inherited differences explain why two people with similar diets, hygiene habits, and hormone levels can have vastly different skin.
How Diet Fuels the Cycle
Two dietary patterns have the strongest evidence linking them to acne: high-glycemic foods and dairy.
When you eat foods that spike your blood sugar quickly (white bread, sugary drinks, processed snacks), your pancreas releases a large burst of insulin. That insulin triggers a cascade that raises levels of a growth hormone called IGF-1. IGF-1 does several things at once that are bad for acne-prone skin. It stimulates oil glands to produce more sebum. It encourages the skin cells lining your pores to multiply faster. And it amplifies the effect of androgens by making androgen receptors more active. In short, a high-glycemic diet pushes all four acne mechanisms harder.
Dairy has its own connection, and the data is surprisingly specific. A meta-analysis of observational studies found that people who consumed the most dairy overall had 2.6 times the odds of developing acne compared to those who consumed the least. Skim milk carried higher risk (1.8 times the odds) than low-fat milk (1.25 times). Interestingly, yogurt and cheese showed no significant association. The leading theory is that milk contains its own hormones and growth factors, including IGF-1, that survive digestion and add to the hormonal signals your oil glands are already receiving. Why skim milk appears worse than whole milk isn’t fully settled, but one explanation is that processing changes the balance of bioactive compounds.
The Stress Connection
The link between stress and breakouts isn’t just anecdotal. When you’re under psychological stress, nerve fibers around your oil glands release a signaling molecule called substance P. This neuropeptide directly alters the structure and function of oil glands, increasing their activity. Stress also triggers the release of cortisol and other hormones through the body’s broader stress response, which can amplify androgen activity and suppress parts of the immune system that keep skin bacteria in check. The result is a perfect storm: more oil, more inflammation, and a less balanced skin microbiome, all arriving at the same time your body is least equipped to deal with them.
Why Some Pimples Stay Mild and Others Scar
The severity of any given breakout depends largely on the depth of the clog and the intensity of the immune response. A shallow plug near the skin’s surface becomes a blackhead (the dark color is oxidized oil, not dirt) or a whitehead. When the plug sits deeper and bacteria multiply enough to provoke a strong immune response, the follicle wall can rupture beneath the skin’s surface, spreading inflammation into surrounding tissue. This is what produces nodules and cysts, the deep, painful lesions most likely to leave scars.
Your immune system’s particular style of response matters too. Some people’s skin mounts a rapid, intense inflammatory reaction to even small bacterial populations, while others tolerate the same bacterial load with minimal visible inflammation. This variation is partly genetic, partly influenced by the specific bacterial strains present, and partly shaped by environmental factors like diet and stress. It’s the reason acne is so individual: the same underlying mechanisms play out differently depending on the mix of triggers each person carries.