Acne forms when hair follicles in your skin become clogged with oil and dead skin cells, then get inflamed. It’s not a single event but a chain reaction involving four factors: excess oil production, a buildup of skin cells inside the pore, bacterial overgrowth, and inflammation. Understanding each step helps explain why breakouts happen and why so many different triggers, from hormones to diet to medications, can set the process in motion.
What Happens Inside a Pore
Every pore on your face, chest, and back contains a tiny hair follicle connected to an oil gland. These glands produce sebum, an oily substance that normally travels up the follicle and spreads across your skin to keep it moisturized. Acne starts when this system breaks down at any of four stages.
First, the cells lining the inside of the follicle start multiplying faster than they can shed. Instead of rising to the surface and flaking off, they stick together and form a plug. This process, called hyperkeratinization, narrows or blocks the opening of the pore.
Second, the oil gland ramps up sebum production. The extra oil has nowhere to go because the pore is partially or fully blocked, so it pools behind the plug. This creates the initial bump: a whitehead if the pore is sealed, or a blackhead if it’s open to air (the dark color comes from oxidation, not dirt).
Third, a bacterium called Cutibacterium acnes (C. acnes) that normally lives on your skin in small numbers thrives in this oxygen-poor, oil-rich environment. It feeds on the trapped sebum and multiplies rapidly inside the clogged follicle.
Fourth, your immune system detects the bacterial overgrowth and launches an inflammatory response. Immune cells flood the area and release signaling molecules that cause redness, swelling, and pain. This is the step that turns a simple clogged pore into an angry, red pimple, or in more severe cases, a deep cyst.
How Hormones Drive Oil Production
Hormones are the single biggest reason oil glands overproduce sebum. Androgens, a group of hormones that includes testosterone, are the primary drivers. Your body converts testosterone into a more potent form called DHT directly inside the oil gland cells. Once DHT binds to receptors in those cells, it enters the cell nucleus and essentially tells the gland to grow larger and produce more oil.
This is why acne peaks during puberty, when androgen levels surge in both boys and girls. But hormonal acne isn’t limited to teenagers. Women commonly experience flare-ups around their menstrual cycle, during pregnancy, or after stopping birth control, all times when hormone levels shift. Roughly 31% of women in their 30s still have clinically recognizable acne, and prevalence at that age ranges from 26% to 35% across studies. So if you’re dealing with breakouts well past your teen years, you’re far from alone.
The Role of Bacteria and Inflammation
C. acnes doesn’t cause acne simply by being present on your skin. Everyone has it. The problem starts when it gets trapped inside a clogged pore, where it multiplies and releases substances that your immune system recognizes as a threat. Immune cells called macrophages detect bacterial DNA and respond by pumping out inflammatory molecules, including several that cause tissue swelling, redness, and the buildup of pus.
The intensity of your immune response partly explains why two people with the same number of clogged pores can have very different skin. One person might get mostly blackheads and whiteheads, while another develops deep, painful nodules. Genetics play a significant role in how aggressively your immune system reacts to the same bacterial trigger.
How Diet Affects Breakouts
The connection between diet and acne centers on a hormone called insulin-like growth factor 1 (IGF-1). Foods that spike your blood sugar quickly, like white bread, sugary drinks, and processed snacks, cause your body to release more insulin, which in turn raises IGF-1 levels. IGF-1 stimulates oil production and skin cell growth, both of which feed the acne cycle.
In a randomized controlled trial, participants who switched to a low glycemic diet (foods that raise blood sugar slowly, like whole grains, legumes, and most vegetables) saw their IGF-1 levels drop significantly in just two weeks. The effect was specific to IGF-1; insulin and blood sugar levels didn’t change meaningfully between groups in that short window. This suggests that the dietary link to acne works through a particular hormonal pathway rather than through blood sugar alone.
Dairy, especially skim milk, has also been consistently associated with acne in observational studies. The proposed mechanism is similar: dairy contains hormones and bioactive molecules that can raise IGF-1 levels.
Medications That Trigger Acne
Several common medications can cause or worsen breakouts, and the resulting acne often looks slightly different from typical acne. Corticosteroids are one of the most frequent culprits, whether taken orally, inhaled for asthma, or applied heavily as a topical cream. Steroid-induced breakouts tend to appear as uniform, small bumps rather than the mixed blackheads and cysts of hormonal acne.
Other medications with well-established links to acne include:
- Lithium, used for mood disorders, which causes skin problems with long-term use
- Anabolic steroids and testosterone, which directly increase androgen activity in the skin
- High-dose vitamin B12 supplements (5 to 10 mg per week), which have triggered breakouts in multiple case reports
- Certain progestins in older birth control formulations, particularly first-generation types like levonorgestrel, which have testosterone-like effects
- Isoniazid, a tuberculosis drug, especially in combination with other TB medications
Even fluoride in toothpaste has been linked to acne-like eruptions around the mouth. If your breakouts started shortly after beginning a new medication or supplement, that timing is worth noting.
Environmental and Lifestyle Factors
Air pollution contributes to acne in ways researchers are still mapping out. Particulate matter, the tiny particles in smog and exhaust, appears to change the composition of sebum on your skin. Exposure to polluted air has been shown to increase sebum output while depleting protective antioxidants like vitamin E from the skin’s surface. The result is oil that’s more prone to oxidation and pore-clogging, combined with increased inflammatory signaling.
Friction and pressure on the skin also matter. Tight helmets, chin straps, backpack straps, and even the habit of resting your face on your hands can create localized breakouts by trapping sweat and oil against the skin. This type, sometimes called mechanical acne, is common in athletes and people who wear protective gear regularly.
Stress doesn’t directly cause acne, but it elevates cortisol, which can increase oil production and slow wound healing. Chronic stress tends to make existing acne worse and slower to clear rather than creating breakouts from scratch.
Why Some People Get Acne and Others Don’t
Genetics set the baseline. If both of your parents had acne, your chances of developing it are significantly higher. What you inherit isn’t a single “acne gene” but a collection of traits: how large your oil glands are, how sensitive they are to hormones, how quickly your skin cells turn over, and how reactive your immune system is to C. acnes bacteria. Two people can have the same diet, stress level, and skincare routine, and one gets clear skin while the other breaks out, largely because of these inherited differences.
Skin type also plays a role. Oilier skin provides more fuel for the acne cycle, which is why breakouts concentrate on the face, chest, and upper back, areas with the highest density of oil glands. People with naturally dry skin can still get acne, but it’s less common and usually less severe.