What is acne? How a pimple forms

Acne is a skin condition that starts inside a pore. Oil and dead skin cells stick together and block the opening, and what happens after that decides which type of spot you see. This guide explains that process in plain words, who gets acne, how long it lasts, and when it needs a doctor. It is part of Skinli's guide to the types of acne, which covers each spot type and each body area in its own page.

Skinli is an education site and is not a doctor. Every medical statement below names its source, and the sources are listed at the end of the page.

What acne is, in one paragraph

The American Academy of Dermatology (AAD) defines acne as a skin condition that can cause one or more of four things: blackheads, whiteheads, pimples, and acne cysts or nodules, which it describes as deep, painful breakouts. The full medical name is acne vulgaris. MedlinePlus, the health library of the US National Library of Medicine, lists pimples and zits as everyday names for the same condition.

NIAMS, a part of the US National Institutes of Health, gives a short definition: acne happens when hair follicles under the skin become clogged. A hair follicle is the small tube in the skin that holds each hair, and its opening at the surface is what people call a pore.

How a pimple forms, step by step

Every hair follicle is attached to an oil gland. MedlinePlus explains that the oil from this gland helps remove old skin cells and keeps skin soft. In healthy skin, oil flows up the follicle and out onto the surface, and old skin cells shed without trouble.

NIAMS describes what goes wrong in acne. The hair, the oil and the skin cells that line the follicle stick together inside the pore. The cells cannot shed, and the oil cannot reach the surface. The result is a plug.

From there, the plug can go four ways:

What happens to the plug	What you see	Medical name
The opening closes over it	A small white or skin-colored bump	Whitehead, or closed comedo
The opening stays wide and the plug meets the air	A dark dot	Blackhead, or open comedo
Bacteria grow in it and the area swells	A red bump, with or without pus	Papule (no pus) or pustule (pus)
The swelling goes deep into the skin	A large, sore lump	Nodule or cyst

The AAD explains why the swelling happens. The bacteria that live on everyone's skin can multiply quickly in the extra oil. As the pore fills with bacteria, swelling develops and a pimple appears. MedlinePlus adds that the body's defense system reacts to the trapped bacteria, which causes the red, sore bump.

NIAMS describes the last step. When the wall of a plugged follicle breaks down, it spills bacteria, skin cells and oil into the skin around it. That is when a pimple forms.

"Comedo" is the medical word for a clogged pore, and blackheads and whiteheads together are called comedones. The same root is in the label word non-comedogenic, which the AAD lists with "oil-free" and "won't clog pores" as terms for products that avoid clogging pores. The blackheads vs whiteheads guide goes into the two plug types in detail.

Where acne shows up

The NHS says acne usually affects the face, chest and back. NIAMS adds the shoulders. MedlinePlus lists the face and shoulders as the most common places and says acne may also appear on the trunk, arms, legs and buttocks.

The areas people search for most each have their own page in this hub: back acne, chest acne, butt acne, a pimple on the lip, a pimple on the nose, and forehead and chin acne.

Who gets acne

Almost everyone gets some. The AAD says nearly everyone develops at least a few breakouts during the teenage years. NIAMS says people of all races and ages get acne, and that it is most common in teens and young adults.

The pattern changes with age:

• In the teenage years, NIAMS says acne is more common in males.
• In adults, NIAMS says acne is more common in women.
• The AAD says 20 to 40 percent of women who have adult acne get it for the first time as an adult.
• The AAD says 20 percent of newborns develop a type of acne called neonatal acne, which goes away on its own without scars.

The 2024 acne guideline published in the Journal of the American Academy of Dermatology says acne commonly affects adults, teenagers, and children aged 9 or older.

What sets off acne in one person and not another is covered in the separate guide on what causes acne. In short, the AAD, NIAMS and the NHS all point to hormones, family history, some medicines and oily products.

How acne looks on different skin tones

The NHS describes papules as raised bumps that look pink or red on white skin and may look brown or purple on brown and black skin. It also says the redness of a skin infection can be harder to see on brown and black skin.

What acne leaves behind can differ too. The AAD says that as a breakout clears, some people see a flat spot where the acne was. The spot can be pink, red, purple, black or brown. These spots clear on their own, but the AAD says some last a year or longer, and the darker the spot, the longer it takes. The medical name is postinflammatory hyperpigmentation. Dark marks after pimples covers the difference between these flat marks and true scars.

How long acne lasts

Acne can last for years. The AAD gives these figures:

• Most teenage boys with acne keep getting breakouts until they are 20 to 25 years old.
• A teenage girl can expect her acne to last into her 20s or longer.
• 5 percent of women still have acne in their 40s.

NIAMS says acne usually goes away by the thirties, but some people in their forties and fifties still have it.

Why doctors recommend treating it

The AAD says acne will go away on its own in the end, and it still recommends treatment for four reasons. You avoid years of breakouts. You stop acne from getting worse, because a few pimples can turn into widespread blackheads, whiteheads and deep, painful pimples. You protect your mood, because studies link severe acne with higher rates of depression and anxiety. And you lower the chance of scars, because all types of acne, from blackheads to deep nodules, can scar.

Treatment takes time. The AAD says you need to use the same treatment for 6 to 8 weeks to see improvement. Skinli's plan runs for 12 weeks, and how long an acne routine takes to work explains what to look for at each stage.

For the steps of a daily routine, the complete acne skincare routine covers cleanser, treatment, moisturizer and sunscreen, morning and night.

Things acne is often confused with

Several other skin conditions produce bumps that look like pimples. The AAD has a page on this, and the list includes rosacea, keratosis pilaris, hidradenitis suppurativa, perioral dermatitis, and polycystic ovary syndrome, a hormone condition that often comes with acne. The AAD also has a page on folliculitis, an infection of the hair follicles that usually looks like a sudden acne breakout.

One look-alike gets its own page in this hub. Fungal acne is an overgrowth of yeast in the hair follicles. It itches, its bumps all look the same, and a review in the Journal of Fungi (2025) says it is often misdiagnosed as acne.

The AAD gives clues for each look-alike, such as small bumps that burn or itch only around the mouth, or deep lumps where skin touches skin. If your spots match one of them, the AAD says a dermatologist can give you the right diagnosis and treatment.

When acne needs a doctor

The NHS says to see a GP, a family doctor, if any of these apply:

• You have tried treating acne yourself and it is not getting better.
• You have lots of spots, or the spots are deep, painful or causing scars.
• You have signs of a skin infection, such as hot, swollen and red skin.
• Acne is affecting your mental health.

The AAD adds that anyone with deep, painful acne needs a dermatologist's help, because store-bought products cannot clear it. The cystic acne guide explains what doctors offer for those deep lumps.

If you are not sure which type of spots you have, Skinli's photo check looks at one face photo, asks two questions, and builds a starting plan that says when to see a doctor.