Acne is a skin condition where hair follicles become clogged with oil and dead skin cells, leading to pimples, blackheads, and deeper lumps on the face, chest, and back. It affects roughly 1 in 5 people overall, with the highest rates among 16- to 24-year-olds, where prevalence reaches about 28%. While most people associate acne with teenagers, about 1 in 5 adults between 25 and 39 also deal with it.
How Acne Forms
Acne develops through four connected processes happening inside your pores. First, skin cells lining the hair follicle multiply faster than normal and don’t shed properly. Instead of sloughing off, they clump together and form a tiny plug deep in the pore. Second, oil glands attached to the follicle produce more sebum (the skin’s natural oil) than the pore can handle. That excess oil gets trapped behind the plug.
Third, a bacterium that naturally lives on everyone’s skin begins to thrive in this oil-rich, oxygen-poor environment. As it feeds on trapped sebum, it releases enzymes and byproducts that irritate the follicle wall. This triggers the fourth step: your immune system sends inflammatory cells to the area, producing the redness, swelling, and tenderness you recognize as a breakout. All four factors work together, which is why effective treatment usually targets more than one of them.
Types of Acne Lesions
Not all pimples look or feel the same. Acne lesions fall into two broad categories: non-inflammatory and inflammatory.
Non-inflammatory acne includes comedones, which are simply clogged pores. A closed comedone (whitehead) sits beneath a thin layer of skin and appears as a small flesh-colored bump. An open comedone (blackhead) reaches the skin’s surface, where the trapped material oxidizes and turns dark. Neither type is typically painful or red.
Inflammatory acne is more noticeable and uncomfortable:
- Papules are small, tender red bumps where the follicle wall has become irritated.
- Pustules are similar bumps with a white or yellow center filled with pus, the classic “squeezable” spot.
- Nodules are large, painful lumps that form deeper in the skin and can linger for weeks.
- Cysts are soft, fluid-filled swellings beneath the surface, often the most painful type and the most likely to leave scars.
Mild acne generally involves fewer than five or six inflammatory spots on each side of the face, while moderate acne falls in the range of six to twenty. Severe cases involve more than twenty inflammatory lesions per half-face, often including nodules or cysts.
What Causes Breakouts
Hormones
Rising levels of testosterone during puberty are the main reason acne peaks in the teenage years. Testosterone stimulates oil glands to produce far more sebum than the skin actually needs. This happens in both boys and girls, though boys tend to produce more testosterone and often get more severe acne as a result.
Women are more likely than men to experience adult acne, largely because of hormonal shifts at specific times. Many women notice flare-ups just before their period, during the first trimester of pregnancy, or alongside conditions like polycystic ovary syndrome (PCOS), which alters hormone balance.
Genetics
Acne runs in families. If both your parents had acne, you’re more likely to develop it earlier and more severely. If one or both parents dealt with acne as adults, you’re also at higher risk of adult breakouts. Genetics influence how sensitive your oil glands are to hormones, how quickly your skin cells turn over, and how aggressively your immune system responds to clogged pores.
Medications
Certain drugs can trigger or worsen acne as a side effect. Steroid medications (including corticosteroids and anabolic steroids), lithium, and some epilepsy drugs are well-known culprits. If you notice a breakout pattern that started around the same time as a new medication, that connection is worth raising with whoever prescribed it.
The Role of Diet
Diet doesn’t cause acne on its own, but certain foods can make breakouts worse. High-glycemic foods, like white bread, sugary drinks, and processed snacks, cause rapid spikes in blood sugar. Those spikes trigger inflammation throughout the body and prompt the skin to produce more oil. Both of those responses feed the acne cycle. Following a lower-glycemic diet, one built around whole grains, vegetables, and proteins that digest more slowly, may help reduce breakouts for some people.
Cow’s milk is a more surprising trigger. Studies have linked all types of cow’s milk, including whole, low-fat, and skim, to increased acne breakouts. The exact reason is still unclear, but one theory points to hormones naturally present in milk that promote inflammation. Interestingly, dairy products made from milk, like yogurt and cheese, have not shown the same link in studies.
Marks and Scars After Acne Clears
Even after a breakout heals, it can leave visible marks. The most common is post-inflammatory hyperpigmentation: flat, darkened patches where a pimple used to be. This happens because inflamed skin produces extra melanin, the pigment that gives skin its color, as part of the healing process. These dark spots are not true scars. They don’t damage the skin structure and typically fade on their own over several months, though they can persist longer on darker skin tones.
True acne scars are a different story. They form when a breakout destroys tissue beneath the surface. Pitted or “ice pick” scars result from tissue loss, leaving small depressions in the skin. Raised, thickened scars form when the body overproduces collagen during healing. Nodules and cysts carry the highest scarring risk, which is one reason dermatologists treat severe acne more aggressively. Picking or squeezing any type of acne lesion also significantly increases the chance of permanent scarring.
Who Gets Acne and When
Acne is one of the most common skin conditions worldwide. A large global study published in the Journal of the American Academy of Dermatology found an overall prevalence of about 20.5% across all ages. The 16-to-24 age group had the highest rate at 28.3%, but acne is far from exclusive to that age range. The same study confirmed a growing pattern of adult acne, with roughly 20% of people between 25 and 39 receiving an acne diagnosis.
While puberty remains the most common trigger, adult-onset acne can appear for the first time in your 20s, 30s, or even later. Women make up the majority of adult acne cases, often related to the hormonal fluctuations described above. For many adults, acne that started in adolescence simply never fully resolved, continuing at a lower level into their 30s and beyond.