Acne develops when tiny hair follicles in your skin become clogged with oil and dead skin cells, then get inflamed. But the reasons that process kicks off in the first place range from hormones and genetics to diet, medications, and even your skincare products. Most people have more than one factor at play, which is why acne can be so persistent and personal.
How Acne Forms in the Skin
Every pore on your face, chest, and back contains a tiny oil-producing gland. These glands sit at the base of hair follicles and release an oily substance called sebum that normally travels up through the pore and onto the skin’s surface, keeping it moisturized. Acne starts when this system breaks down in one of several ways.
First, the cells lining the inside of the follicle can multiply too quickly and stick together instead of shedding normally. This creates a plug that traps oil inside. Second, the oil glands themselves may produce too much sebum, adding pressure behind the blockage. Third, a bacterium that naturally lives on everyone’s skin begins to thrive in the trapped oil. Your immune system responds to the growing bacterial population by flooding the area with inflammatory signals, turning what was a simple clogged pore into a red, swollen, sometimes painful lesion. These four steps (abnormal skin cell turnover, excess oil, bacterial overgrowth, and inflammation) work together in virtually every case of acne.
Hormones and Oil Production
Hormones are the single biggest driver of how much oil your skin produces. Androgens, a group of hormones that includes testosterone, directly stimulate oil glands to enlarge and secrete more sebum. This is why acne so often appears during puberty, when androgen levels rise sharply in both boys and girls. Early research showed that prepubertal boys given testosterone developed bigger oil glands and measurably higher sebum output.
Hormonal acne doesn’t end with adolescence. Many women experience breakouts around their menstrual period, during pregnancy, or after starting or stopping birth control. Interestingly, most women with acne have perfectly normal androgen levels in their blood. Researchers believe their oil glands are simply more sensitive to androgens, or that the glands themselves convert weaker hormones into more potent forms locally, right at the skin level.
Polycystic ovary syndrome (PCOS) is a common condition that drives androgen levels higher than normal in women, and acne is one of its hallmark symptoms. A meta-analysis of 60 studies found that 42% of adult women with PCOS had acne, and the rate was even higher in adolescents at 59%. If your breakouts come alongside irregular periods, thinning hair on the scalp, or excess body hair, PCOS may be worth investigating.
Genetics Play a Major Role
If your parents had significant acne, your odds of developing it are high. A twin study published in the Journal of Investigative Dermatology found that 81% of the variation in acne severity was attributable to genetic factors. Only 19% came from environmental influences. That’s a remarkably strong genetic component, on par with conditions like height or certain autoimmune diseases.
What you inherit isn’t acne itself but the biological tendencies that lead to it: how large your oil glands are, how your immune system responds to skin bacteria, and how your follicle cells shed. This genetic load explains why some people break out despite doing “everything right” with their skincare, while others rarely wash their face and never get a pimple.
Bacteria and the Immune Response
A microbe called Cutibacterium acnes (C. acnes) lives naturally on nearly everyone’s skin. It feeds on sebum and is usually harmless. Problems start when a clogged pore creates a low-oxygen, oil-rich environment where C. acnes can multiply rapidly.
As the bacterial population grows, your immune system detects it through receptors on the surface of immune cells. These receptors trigger the release of a cascade of inflammatory molecules that cause redness, swelling, and pus formation. Inflamed acne lesions have significantly higher levels of these inflammatory signals compared to the clear skin right next to them. This is why acne isn’t just a cosmetic issue of clogged pores. It’s an active immune response happening inside your skin, and the inflammation itself can cause lasting damage in the form of dark marks or scars.
Diet: Glycemic Load and Dairy
The link between food and acne was dismissed for decades, but recent evidence has brought it back into focus. Two dietary patterns consistently show up in the research: high-glycemic diets and dairy consumption.
Foods that spike your blood sugar quickly (white bread, sugary drinks, processed snacks) trigger a hormonal chain reaction. Your body releases insulin, which in turn raises levels of a growth factor called IGF-1. IGF-1 stimulates oil production and skin cell growth, both of which feed acne. A randomized controlled trial found that people with moderate to severe acne who followed a low-glycemic diet for just two weeks saw a significant drop in their IGF-1 levels. That’s a short window to see measurable biological change, suggesting the diet-acne connection is real and responsive.
Dairy, particularly milk, also appears to increase acne risk. A meta-analysis from Johns Hopkins found that all types of milk were associated with higher odds of acne, but skim milk carried the strongest association, followed by low-fat and then full-fat. The reason likely isn’t the fat content. Milk naturally contains hormones and growth factors that may amplify the same IGF-1 pathway. Cheese and yogurt haven’t shown as strong a link, possibly because fermentation alters some of those compounds.
Medications That Trigger Breakouts
Certain medications can cause acne or make existing breakouts worse. Drug-induced acne often looks slightly different from regular acne: the bumps tend to be uniform in size and appear suddenly, often on the chest and back as well as the face.
The most well-established culprits include:
- Corticosteroids: Oral, inhaled, or heavily applied topical steroids are among the most common triggers. Steroid acne can appear within weeks of starting treatment.
- Anabolic steroids and testosterone therapy: These directly increase androgen activity in the skin. Bodybuilders using anabolic steroids commonly develop severe acne on the back and shoulders.
- Lithium: Used for mood disorders, lithium is a well-documented acne trigger.
- B vitamins: High-dose vitamin B12 supplements (5 to 10 mg per week) have been linked to acne flares.
- Certain cancer drugs: A class of targeted therapies called EGFR inhibitors causes acne-like rashes in roughly 85% of patients.
- Some progestins: Older-generation progestins found in some birth control pills have androgenic effects that can worsen acne, while newer formulations tend to improve it.
If your acne started or worsened shortly after beginning a new medication, that timing is worth noting and discussing with whoever prescribed it.
Skincare and Cosmetic Products
The products you put on your skin to look or feel better can sometimes backfire. Certain ingredients physically block pores, leading to a specific pattern called cosmetic acne. It tends to show up as small, persistent bumps rather than deep, inflamed cysts, and it concentrates wherever you apply the offending product.
A review in Dermatology Times identified some of the most common pore-clogging ingredients currently found in everyday products. In facial cleansers, certain harsh surfactants along with lauric acid and stearic acid were the biggest offenders. In moisturizers, glyceryl stearate was the most frequently identified comedogenic ingredient. These ingredients aren’t inherently bad for everyone, but if you notice breakouts that map to where you apply a particular product, checking its ingredient list against known comedogenic compounds can be revealing. Switching to products labeled “non-comedogenic” often resolves this type of acne within a few weeks.
Stress and Lifestyle Factors
Stress doesn’t cause acne on its own, but it reliably makes existing acne worse. When you’re under psychological stress, your body produces cortisol and other stress hormones. These hormones stimulate oil glands in much the same way androgens do. Students during exam periods, for instance, consistently report worsening breakouts, and studies tracking sebum production confirm the biological mechanism behind it.
Friction and pressure on the skin, sometimes called mechanical acne, can also trigger breakouts. Tight helmets, chin straps, headbands, and even the habit of resting your face on your hands can create localized acne by trapping heat and pressing oil and dead cells deeper into pores. This is common in athletes and anyone who wears tight-fitting gear regularly. The fix is straightforward: reduce the friction, clean the area after sweating, and keep gear clean.