Pimples form when oil and dead skin cells get trapped inside a hair follicle, creating a plug that can become inflamed or infected. This process involves four key factors working together: clogged pores, excess oil production, bacterial activity, and your immune system’s inflammatory response. Understanding each step explains not just why pimples happen, but why some people get them worse than others.
How a Pimple Forms Step by Step
Every pimple starts with a clogged pore. Your skin constantly sheds dead cells, and normally those cells slough off without issue. But sometimes the cells lining a hair follicle multiply too fast and don’t shed properly, creating a tiny plug of skin cells and oil deep in the pore. This microscopic blockage, called a microcomedone, is the seed of every pimple, whether it becomes a small whitehead or a painful cyst.
At the same time, the oil glands attached to that follicle may be pumping out more sebum (your skin’s natural oil) than the pore can handle. Sebum normally travels up the follicle and spreads across your skin’s surface to keep it moisturized. When a plug blocks the exit, oil backs up behind it. The follicle swells, and bacteria that normally live harmlessly on your skin now have a sealed, oxygen-free environment with plenty of oil to feed on.
The bacterium most involved is one that naturally lives in your pores. When it multiplies inside a clogged follicle, your immune system detects it and sends white blood cells to fight the infection. That immune response is what causes the redness, swelling, and tenderness you see and feel. The pus inside a pimple is largely made up of dead white blood cells, dead bacteria, and cellular debris from this battle.
Why Hormones Are the Biggest Driver
Oil production in your skin is directly controlled by androgens, a group of hormones that includes testosterone. This is why acne typically kicks in during puberty, when androgen levels surge in both boys and girls. Higher androgens signal oil glands to grow larger and produce more sebum, setting the stage for clogged pores.
Your oil glands contain enzymes that convert weaker hormones into more potent forms right inside the gland itself. Skin that’s prone to acne has higher levels of these converting enzymes than skin in areas that rarely break out. This local hormone processing explains why pimples cluster on your face, chest, and back, where oil glands are largest and most dense, while the skin on your shins stays clear.
Hormonal acne is especially well-documented in women. Studies published in JAMA Dermatology found that women with acne had significantly higher blood levels of several androgens compared to women without acne. This hormonal connection also explains breakouts tied to menstrual cycles, polycystic ovary syndrome, and the transition off birth control. In prepubertal girls, the earliest acne often correlates with rising levels of a specific adrenal hormone that serves as a building block for testosterone.
Genetics Play a Major Role
If your parents had acne, your chances of getting it are substantially higher. A twin study of women found that 81% of the variation in acne severity was attributable to genetic factors. That leaves only about 19% explained by environment, diet, skincare habits, and other external influences. Genetics likely determine how large your oil glands are, how your skin cells behave inside follicles, and how aggressively your immune system responds to clogged pores.
Different Types of Pimples
Not all pimples are the same, and the type you get reflects how deep the blockage is and how much inflammation is involved.
- Whiteheads (closed comedones): Small, flesh-colored bumps where the plug stays sealed beneath the skin’s surface. These are non-inflammatory, meaning they aren’t red or painful.
- Blackheads (open comedones): The follicle opening is wide enough that the plug is exposed to air. The dark color isn’t dirt. It’s the result of oxidation, similar to how a cut apple turns brown.
- Papules: Red, tender bumps that form when the follicle wall breaks under pressure and the immune system moves in. There’s no visible pus yet.
- Pustules: The classic “pimple” with a white or yellow center of pus sitting on a red base.
- Cysts and nodules: Deep, fluid-filled or solid lesions that form when inflammation extends far below the skin surface. These are the most painful type and the most likely to leave permanent scars.
Whiteheads and blackheads can persist for weeks or months if nothing dislodges the plug. Papules and pustules tend to cycle through in two to six weeks. Cysts and nodules can last months or even years without treatment and carry a much higher risk of scarring.
How Diet Fits In
The link between food and acne is real but more nuanced than most people think. The strongest evidence points to high-glycemic foods, things that spike your blood sugar quickly like white bread, sugary drinks, and processed snacks. When blood sugar rises sharply, your body releases a burst of insulin, which in turn raises levels of other hormones that stimulate oil production. A clinical trial found that patients on a low-glycemic diet for 10 weeks had significant improvement in acne lesions compared to a control group.
Dairy, particularly skim milk, has a more complicated relationship with acne. One study found that teenagers with acne consumed significantly more low-fat and skim milk than those without acne, but found no difference for full-fat milk or total dairy intake. The proposed explanation is that milk triggers an insulin response three to six times higher than you’d expect based on its sugar content alone, and it contains growth factors that may stimulate oil glands. However, the evidence isn’t strong enough for dermatologists to make blanket dietary recommendations about dairy. If you suspect milk is contributing to your breakouts, eliminating it for a few months is a reasonable experiment.
Other Common Triggers
Beyond hormones, genetics, and diet, several everyday factors can push your skin toward breakouts. Stress increases cortisol, which can amplify oil production. Friction and pressure from phone screens, helmets, chin straps, or resting your face on your hands can irritate follicles and worsen clogging in those areas.
Some skincare and cosmetic products contain oils or ingredients that block pores, a property called comedogenicity. Switching to non-comedogenic products can reduce breakouts caused by this route. Over-washing your face or scrubbing aggressively can also backfire by stripping natural oils, prompting your skin to compensate by producing even more sebum.
Certain medications, including corticosteroids, lithium, and some hormonal treatments, can trigger or worsen acne as a side effect. If breakouts coincide with starting a new medication, that connection is worth discussing with whoever prescribed it.
Why Some People Get Acne and Others Don’t
The frustrating reality is that acne involves a combination of factors you can control and many you can’t. Someone with genetically large oil glands, sensitive hormone receptors, and an aggressive inflammatory response will break out more easily than someone without those traits, even if both people eat the same diet and follow the same skincare routine. The 81% heritability figure underscores this: your DNA sets the baseline, and environmental factors nudge the outcome up or down from there.
This also explains why acne peaks during puberty but can persist or reappear well into adulthood, particularly in women whose hormone levels shift during menstrual cycles, pregnancy, or perimenopause. Adult acne isn’t a hygiene problem or a sign you’re doing something wrong. It’s a reflection of the same biological machinery, just activated by different triggers at different life stages.