Acne develops when hair follicles in your skin become clogged with oil and dead skin cells, then get inflamed. But that simple explanation hides a chain of events driven by hormones, bacteria, genetics, and sometimes your environment. Four factors work together to produce breakouts: excess oil production, clogged pores, bacterial colonization, and inflammation. Understanding which of these factors is most active in your skin helps explain why some treatments work and others don’t.
The Four Factors Behind Every Breakout
Every acne lesion, from a barely visible bump to a deep cyst, traces back to the same sequence inside a hair follicle. First, your skin’s oil glands produce too much sebum. Second, dead skin cells that normally shed from the follicle lining instead stick together and form a plug. This combination of oil and dead cells creates a microcomedone, a tiny blockage invisible to the naked eye. Third, a bacterium called Cutibacterium acnes that normally lives on your skin thrives inside that sealed, oily environment. Fourth, your immune system reacts to the bacterial byproducts, sending white blood cells that cause redness, swelling, and pus.
What makes this process surprising is the timing. Researchers biopsying skin that looked completely clear found elevated levels of immune cells (T cells and macrophages) around follicles that hadn’t yet formed a visible clogged pore. That means inflammation isn’t just a consequence of acne. It’s part of the process from the very beginning, even before a pore visibly clogs.
When the inflammation stays mild, you get a small red bump (papule). When it intensifies, the follicle wall ruptures beneath the skin’s surface, spilling its contents into surrounding tissue. Your immune system escalates its response, and the result is a deeper, more painful nodule or cyst. Squeezing or scrubbing aggressively can accelerate this rupture, which is why dermatologists consistently warn against picking.
Why Hormones Are the Primary Driver
Androgens, a group of hormones that includes testosterone, are the main signal telling your oil glands to ramp up production. During puberty, androgen levels surge in both boys and girls, which is why acne typically begins in the early teenage years. Your sebaceous glands actually contain the enzymes needed to convert weaker hormones into more potent forms of testosterone locally, meaning oil glands can amplify hormonal signals on their own.
This hormonal connection also explains adult acne. Many women experience breakouts that flare in a predictable pattern tied to their menstrual cycle, typically in the week before a period when progesterone and androgens shift relative to estrogen. The breakouts tend to cluster along the jawline, chin, and lower cheeks.
Polycystic ovary syndrome (PCOS) is one of the more common hormonal conditions linked to persistent adult acne. In PCOS, the ovaries produce higher levels of testosterone and a precursor hormone called DHEA. These hormones push sebaceous glands to make more oil while also slowing the rate at which skin cells turn over, making clogs more likely. Acne from PCOS tends to be deeper and more inflamed, often appearing as cystic lesions that resist standard over-the-counter treatments. If topical products aren’t making a dent, unaddressed hormonal imbalance may be the reason.
Genetics Set the Baseline
A large twin study found that 81% of the variation in acne severity was attributable to genetic factors. If both of your parents had significant acne, your risk is substantially higher than someone whose parents had clear skin. Genetics influence how large your oil glands are, how sensitive they are to hormones, how readily your follicles shed dead cells, and how aggressively your immune system responds to clogged pores. You can’t change your genetic predisposition, but knowing your family history helps set realistic expectations about how much effort your skin may require.
Not All Skin Bacteria Are Equal
C. acnes lives on virtually everyone’s skin, yet not everyone gets acne. Recent research helps explain why. Scientists found that different strains of the same bacterium provoke wildly different immune responses. What made certain strains more inflammatory wasn’t their overall genetic type but whether they carried a specific small piece of DNA called a plasmid. Strains with this plasmid triggered a much stronger inflammatory response in surrounding skin cells during lab testing.
This means two people could have similar oil production and pore size, but the one colonized by a more inflammatory strain of C. acnes would develop worse breakouts. It also helps explain why antibiotics work for some people: they reduce the population of these more aggressive bacterial strains.
How Diet Influences Acne
The connection between diet and acne was dismissed for decades, but a growing body of evidence links certain foods to breakouts through a specific hormonal pathway. Dairy products, particularly milk, contain both casein and whey proteins that raise levels of insulin and insulin-like growth factor 1 (IGF-1) in your blood. IGF-1 stimulates oil production in your sebaceous glands through the same signaling pathway that androgens use. Dairy also naturally contains small amounts of hormones, including androgens, which may add to the effect. A meta-analysis of observational studies confirmed a positive association between dairy intake and acne.
High-glycemic foods, those that spike your blood sugar quickly (white bread, sugary drinks, processed snacks), trigger a similar cascade. The resulting insulin spike raises IGF-1, which promotes both oil production and the kind of skin cell overgrowth that plugs pores. This doesn’t mean a single slice of pizza causes a breakout. It means a consistently high-glycemic diet can keep IGF-1 chronically elevated, creating conditions where acne thrives.
Physical Friction and Pressure
A distinct form called acne mechanica develops when something repeatedly rubs, presses, or traps heat against your skin. The friction irritates hair follicles and combines with sweat to accelerate clogging. Common culprits include football helmets, tight bra straps, shirt collars, baseball caps, backpack straps, and even a habit of resting your chin in your hands. Truck drivers and athletes are particularly prone. The breakouts appear wherever the pressure is applied, which helps distinguish this from hormonal acne. Switching to looser gear, showering promptly after sweating, and reducing the friction source often clears it without any topical treatment.
Medications That Trigger Breakouts
Certain medications cause acne-like eruptions as a side effect. Corticosteroids are among the most common offenders, whether taken orally, injected, or applied topically for long periods. The resulting breakouts, sometimes called steroid acne, tend to appear as uniform small bumps rather than the mixed blackheads, whiteheads, and cysts of typical acne.
Other medications linked to acne-like eruptions include lithium, certain cancer therapies that target growth factor receptors, vitamin B12 supplements, and some antibiotics (paradoxically, given that other antibiotics treat acne). Chemicals containing iodides and bromides can also provoke breakouts. If you notice new skin eruptions after starting a medication, the timing is worth mentioning to your prescriber, since the treatment approach for drug-induced breakouts differs from conventional acne.
Why Some People Get Acne and Others Don’t
Acne is rarely caused by a single factor acting alone. It’s the overlap of several contributors that determines whether you break out, how severely, and where on your body. Someone with genetically large oil glands, a hormone shift from puberty or PCOS, a colony of particularly inflammatory bacteria, and a high-dairy diet has multiple forces converging on the same follicles. Someone else with the same genetics but lower androgen levels and a less inflammatory bacterial profile might never develop more than an occasional whitehead.
This layered causation is also why acne treatment often requires addressing more than one factor at a time. Reducing oil production helps, but so does preventing the dead-skin plugs, controlling bacterial overgrowth, and calming inflammation. The most effective approaches typically target at least two of these four pillars simultaneously.