Pimples form when oil, dead skin cells, and bacteria build up inside the tiny openings (pores) in your skin. Every pore connects to a small oil gland underneath the surface, and when that system gets disrupted, a pimple is the result. It’s one of the most common skin conditions on the planet, peaking in the teenage years but persisting well into adulthood for many people.
The process behind a single pimple involves four things happening in sequence: your skin produces too much oil, dead skin cells trap that oil inside the pore, bacteria multiply in the clogged pore, and your immune system launches an inflammatory response. Understanding each step explains not just why pimples happen, but why certain foods, hormones, and habits make them worse.
What Happens Inside a Clogged Pore
Your skin constantly sheds dead cells from its surface. Normally, those cells slough off without you noticing. But sometimes the cells lining the inside of a pore stick together instead of shedding, forming a plug. At the same time, the oil gland attached to that pore keeps pumping out sebum, a waxy substance that normally travels up through the pore to moisturize your skin. When the exit is blocked, sebum pools behind the plug.
This is how you get the earliest form of a pimple: a comedone. If the plug stays beneath the surface, it’s a whitehead. If the plug opens to the air and the trapped material oxidizes, it darkens into a blackhead. Neither of these involves significant inflammation yet. They’re just traffic jams in your pores.
Things escalate when a bacterium called Cutibacterium acnes enters the picture. This microbe lives on everyone’s skin and is normally harmless. But inside a sealed, oil-rich pore, it thrives. As it breaks down sebum, it produces irritating byproducts (free fatty acids) that damage the walls of the pore from the inside. Your immune system detects this damage and sends white blood cells to fight the infection. That’s the redness, swelling, and tenderness you feel. A raised red bump is a papule. Fill it with pus from the immune battle, and it becomes a pustule. If the inflammation goes deep, you get a nodule: a large, hard, painful lump buried under the skin that can take weeks to resolve.
Why Hormones Are the Biggest Driver
The reason acne explodes during puberty comes down to one word: androgens. These hormones, which include testosterone and its more potent form dihydrotestosterone (DHT), surge during adolescence in both boys and girls. Androgens directly stimulate the oil glands in your skin to grow larger and produce more sebum. More oil means more raw material for clogged pores.
DHT is the key player. Your oil glands convert testosterone into DHT right inside the gland itself, and DHT binds to receptors that ramp up oil production at the cellular level. This is why acne tends to cluster in areas with the highest density of oil glands: the face, chest, upper back, and shoulders. In women, androgens from both the ovaries and the adrenal glands contribute, which is why hormonal fluctuations around menstrual cycles, pregnancy, or conditions like polycystic ovary syndrome can trigger breakouts well past the teenage years.
Global data shows that acne prevalence peaks between ages 15 and 19, with females actually carrying a higher burden than males at that age. The condition isn’t just a teenage problem, though. Adult acne, particularly in women, has been rising steadily and is projected to keep increasing through 2050.
How Diet Connects to Breakouts
For decades, dermatologists dismissed the link between diet and acne. That position has shifted. There is now compelling evidence that high-glycemic foods (white bread, sugary drinks, processed snacks) worsen acne through a specific hormonal chain reaction.
When you eat foods that spike your blood sugar quickly, your body releases a surge of insulin. Chronically elevated insulin raises levels of a growth hormone called IGF-1, which is a powerful growth signal for virtually all body tissues. In the skin, IGF-1 does two damaging things at once: it stimulates oil glands to produce more sebum, and it causes the cells lining your pores to multiply faster and stick together. That combination of excess oil and excess dead skin cells is exactly what creates a clogged pore. One clinical trial found that putting people on a low-glycemic diet for 12 weeks lowered their IGF-1 levels and significantly improved their acne.
Milk appears to work through a similar pathway. High milk consumption is associated with a 10 to 20 percent increase in circulating IGF-1 levels among adults, and a 20 to 30 percent increase among children. This holds true for skim milk as well as whole milk, suggesting it’s not the fat content but something else in milk (possibly its own hormones or insulin-stimulating proteins) that drives the effect.
Genetics Set the Stage
If your parents had acne, your odds go up considerably. A twin study found that 81 percent of the variation in acne severity was attributable to genetic factors. That’s a remarkably high heritability, putting acne in the same genetic ballpark as height. What you inherit isn’t acne itself but the traits that make it more likely: larger oil glands, a tendency toward stickier skin cells, or an immune system that overreacts to clogged pores. This is why some people can eat the same diet, go through the same puberty, and have completely different skin.
Friction, Sweat, and Your Environment
Hormones and genetics explain most acne, but your environment can pile on. Heat and humidity increase oil production, and sweat itself creates a moist layer that traps bacteria and debris against the skin. The combination of oil, bacteria, heat, and friction is why breakouts often flare in summer.
Friction deserves special attention. Tight clothing, helmet straps, backpack straps, or anything that rubs repeatedly against your skin can physically push oil and dead cells deeper into pores. This type of acne (sometimes called acne mechanica) shows up along the waistline, chest, back, and anywhere gear presses against you. It can happen year-round if you exercise frequently or wear compression clothing, regardless of the weather.
Why Some Pimples Scar and Others Don’t
The deeper the inflammation, the greater the risk of scarring. Whiteheads and blackheads almost never scar because they involve little to no inflammation. Papules and pustules can leave temporary dark marks, especially on darker skin tones, but these typically fade over weeks to months. Nodules are the ones most likely to leave permanent scars because the inflammation destroys collagen deep in the skin, and the repair process creates uneven tissue.
Picking or squeezing pimples dramatically increases scarring risk. When you rupture a pimple’s wall by pressing on it, you push infected material deeper into surrounding tissue, spreading the inflammation and forcing your immune system into a bigger cleanup job. The more tissue damage, the more scar tissue your body lays down in response.
What Actually Helps
Because pimples form through a chain of events (excess oil, clogged pores, bacterial growth, inflammation), effective treatments target one or more of those steps. Gentle cleansing removes surface oil and debris without stripping the skin so aggressively that oil glands compensate by producing even more sebum. Products containing benzoyl peroxide kill C. acnes bacteria directly. Retinoids (vitamin A derivatives available over the counter and by prescription) speed up skin cell turnover so dead cells are less likely to clump together and block pores.
For hormonally driven acne, especially in adult women, treatments that reduce androgen activity can cut oil production at the source. Dietary changes can also play a supporting role. Shifting toward lower-glycemic foods (whole grains, vegetables, legumes) and reducing milk intake may lower the insulin and IGF-1 signals that feed the cycle. These changes won’t replace targeted skin treatments for moderate or severe acne, but they address one of the upstream triggers that keeps breakouts coming back.
Results from any approach take time. Skin cells cycle over roughly four to six weeks, so a new treatment or habit change needs at least that long before you can judge whether it’s working. Expecting overnight improvement from a process that unfolds over weeks inside each individual pore sets you up for frustration.