Pimples form when a skin pore gets clogged with oil and dead skin cells, creating the perfect environment for bacteria to thrive and trigger inflammation. Four things happen in sequence to produce a pimple: the pore’s lining sheds skin cells too fast, oil glands pump out excess oil, bacteria multiply inside the clog, and the immune system responds with redness and swelling.
How a Pimple Forms Step by Step
Every pore on your skin is the opening of a tiny hair follicle that contains an oil-producing gland. Normally, dead skin cells lining the inside of the pore shed gradually and get pushed to the surface. When something goes wrong, those cells stick together and accumulate faster than they can escape, forming a plug at the top of the pore.
At the same time, the oil gland attached to that follicle may be producing more sebum (the skin’s natural oil) than usual. That oil backs up behind the plug, creating a sealed, oxygen-poor pocket. A bacterium that lives naturally on everyone’s skin thrives in exactly this kind of airless, oil-rich environment. As it multiplies, it releases byproducts that the immune system recognizes as a threat. Your skin cells detect the bacteria through receptor proteins on their surface, which set off a chain of signals that recruit white blood cells, release inflammatory chemicals, and produce the redness, swelling, and tenderness you see as a pimple.
Hormones Are the Biggest Driver
The oil glands in your skin are highly sensitive to hormones, especially androgens like testosterone and its more potent form, DHT. When androgen levels rise, they directly stimulate the oil glands to grow larger and produce more sebum. This is why acne peaks during puberty, when androgen levels surge for the first time, and why it often flares around menstrual periods, during pregnancy, or with conditions like polycystic ovary syndrome.
Growth hormone and insulin-like growth factor (IGF-1) also play a role. Growth hormone promotes the maturation of oil gland cells, pushing them to produce more lipid-rich sebum. Estrogen has the opposite effect, generally slowing oil production, which partly explains why hormonal contraceptives can improve acne in some people.
Genetics Account for Most of the Risk
If your parents had acne, your chances are significantly higher. Twin studies estimate that about 85% of the variation in acne severity is explained by genetics, with only about 15% attributable to environmental factors. That genetic influence likely affects how your oil glands respond to hormones, how quickly your pore linings shed cells, and how aggressively your immune system reacts to bacteria. This is why some people break out constantly while others with similar lifestyles rarely get a pimple.
How Diet Fuels Breakouts
Foods that spike your blood sugar quickly, like white bread, sugary drinks, pastries, and candy, set off a hormonal chain reaction that promotes acne. When blood sugar rises sharply, your body releases a large burst of insulin to bring it back down. That insulin surge does two things: it increases androgen levels, which boost oil production, and it raises IGF-1, which causes the cells lining your pores to multiply faster and makes oil glands produce even more sebum. High blood sugar also appears to push immune cells toward a more inflammatory state, making any existing clog more likely to become a red, swollen pimple rather than staying a quiet blocked pore.
Dairy, particularly milk, has a similar association. Populations that consume little to no dairy and eat low-glycemic diets have notably lower rates of acne. The connection likely comes from the natural hormones and growth factors present in cow’s milk, which can mimic the insulin and IGF-1 signals that stimulate oil production. Ice cream combines both triggers: dairy and high sugar.
Stress Makes It Worse
Stress doesn’t just feel like it causes breakouts. There’s a direct biochemical link. When you’re under psychological stress, your body produces corticotropin-releasing hormone (CRH) and cortisol. Both of these hormones act on your oil glands. CRH is expressed at much higher levels in the oil glands of acne-affected skin compared to clear skin. It stimulates sebum production and activates an enzyme that converts weaker hormones into more potent androgens right there in the skin. So stress essentially gives your oil glands a double signal to ramp up production, which feeds the clogging and bacterial growth cycle that leads to pimples.
Different Types of Pimples
Not all pimples look or feel the same, and the differences come down to how deep the blockage sits and how much inflammation is involved.
- Whiteheads are small, flesh-colored bumps where bacteria and dead skin are trapped inside a completely closed pore. The contents stay sealed beneath the surface.
- Blackheads are essentially whiteheads that have opened up. When the contents of the clogged pore are exposed to air, they oxidize and darken. The black color isn’t dirt.
- Papules are small, solid, raised bumps under the skin with no visible center. They form when a clogged pore becomes inflamed but hasn’t filled with pus yet.
- Pustules are larger, tender bumps with a visible white or yellow center of pus, surrounded by red, inflamed skin. They’re what most people picture when they think of a classic pimple.
- Nodules are hard, painful lumps deep beneath the skin with no visible head. They develop when a clogged, infected pore swells well below the surface.
- Cysts are the most severe form: large, painful, pus-filled lumps deep in the skin. They’re soft to the touch and carry the highest risk of scarring.
Whiteheads and blackheads are non-inflammatory, meaning the immune system hasn’t kicked in yet. Everything from papules onward involves increasing levels of inflammation and deeper involvement in the skin, which is why they hurt more and take longer to heal.
Acne Isn’t Just a Teenage Problem
While acne is most common during adolescence, it’s increasingly recognized in adults. Roughly 1 in 5 people between the ages of 25 and 39 are diagnosed with acne. Adult acne is more common in women, often driven by hormonal fluctuations related to the menstrual cycle, pregnancy, or perimenopause. In many cases, adult breakouts concentrate along the jawline and chin rather than the forehead and nose pattern typical of teenage acne.
Other Common Triggers
Beyond hormones, genetics, diet, and stress, several everyday factors can contribute to breakouts. Friction or pressure on the skin from helmets, tight collars, or resting your face on your hands can push bacteria and oil deeper into pores. Comedogenic (pore-clogging) ingredients in skincare products and makeup are a frequent but overlooked cause, particularly for people who feel like they’re doing everything right but still breaking out. Humidity and sweating can also worsen acne by creating a moist environment that speeds bacterial growth, especially if sweat sits on the skin for extended periods after exercise.
Certain medications, including corticosteroids and some hormonal treatments, can trigger or worsen acne by altering hormone levels or immune function. If breakouts started or worsened after beginning a new medication, that connection is worth exploring.