Dandruff is not a fungus itself, but a fungus is the primary driver behind it. A yeast called Malassezia lives on every human scalp, feeding on the oils your skin produces. In people with dandruff, the byproducts of that feeding process irritate the skin and trigger the flaking, itching, and cell turnover that you see as white flakes. Roughly 50% of adults worldwide deal with dandruff at some point.
How a Scalp Fungus Causes Flaking
Two species of Malassezia, called M. globosa and M. restricta, dominate the scalps of people with dandruff. These yeasts are lipophilic, meaning they feed on fat. Specifically, they release enzymes called lipases that break down the triglycerides in your sebum (the oily substance your scalp naturally produces). The yeast absorbs the saturated fatty acids it needs for energy, but in doing so it leaves behind unsaturated fatty acids, particularly oleic acid, on the surface of your skin.
Oleic acid is the real irritant. When it accumulates on the scalp, it penetrates the outer skin barrier and triggers an inflammatory response in susceptible people. Your scalp reacts by speeding up skin cell turnover, pushing cells to the surface faster than normal. Those cells clump together and shed as the visible white or yellowish flakes you recognize as dandruff. Reducing the fungal population reduces free fatty acid levels, which in turn reduces flaking and itch.
Why Some People Get Dandruff and Others Don’t
Here’s the key detail: Malassezia lives on virtually everyone’s scalp, yet not everyone gets dandruff. The difference comes down to three factors working together: the amount of fungus present, how much sebum your scalp produces, and your individual sensitivity to the irritating fatty acids the yeast leaves behind. Some people’s skin simply reacts more strongly to oleic acid than others.
Sebum production is largely driven by hormones, especially androgens. This is why dandruff often first appears during puberty, when androgen levels rise and oil glands become more active. Growth hormone and stress-related hormones can also increase oil output, giving the yeast more food to work with. The more sebum available, the more the fungus thrives, and the more irritating byproducts accumulate.
The bacterial balance on your scalp also plays a role. Healthy scalps tend to have a higher ratio of one common bacterium (Propionibacterium) relative to another (Staphylococcus). In dandruff-affected scalps, that ratio shifts. Researchers have also found previously uncharacterized Malassezia species appearing in the core fungal community of dandruff scalps, suggesting the microbial picture is more complex than just the two well-known species.
Dandruff vs. Seborrheic Dermatitis
Dandruff and seborrheic dermatitis exist on a spectrum of the same underlying process, but they look and feel different. Dandruff is restricted to the scalp. It shows up as light, white-to-yellow flakes scattered through your hair, with little to no redness and only mild itching. Seborrheic dermatitis is more intense: it produces larger, oilier scales on red, inflamed patches and can spread beyond the scalp to the face (especially around the nose, eyebrows, and eyelids) and upper chest.
Under a microscope, the distinction is clearer. Dandruff involves minimal or no immune cell activity in the skin, while seborrheic dermatitis shows significant inflammation with immune cells clustering around blood vessels and hair follicles. Both conditions involve Malassezia, but the body’s inflammatory response is far more aggressive in seborrheic dermatitis. If your flaking comes with visible redness, greasy patches, or extends beyond your scalp, you’re likely dealing with seborrheic dermatitis rather than simple dandruff.
How Antifungal Shampoos Work
Because a fungus drives the process, the most effective dandruff treatments are antifungal. The most common active ingredient in over-the-counter dandruff shampoos, zinc pyrithione, attacks Malassezia through multiple pathways at once. It floods yeast cells with excess zinc, disrupts their ability to produce energy in their mitochondria, and, critically, suppresses the very lipase enzymes the yeast uses to break down sebum. Fewer lipases means less oleic acid on your scalp, which means less irritation and flaking.
Other antifungal options include ketoconazole and selenium sulfide. In a clinical trial of 246 patients with moderate to severe dandruff, both ketoconazole 2% and selenium sulfide 2.5% shampoos significantly outperformed placebo at every visit. Ketoconazole showed a slight edge in the first week and was better tolerated, with none of the adverse effects reported by some selenium sulfide users. Improvement can begin within the first week of regular use.
The catch is that dandruff is a chronic, relapsing condition. Antifungal shampoos control the yeast population, but Malassezia never truly leaves your scalp. When you stop treatment, the fungus rebounds, sebum breakdown resumes, and flaking returns. Most people find they need to use medicated shampoo on an ongoing basis, sometimes alternating with a regular shampoo to find a maintenance routine that keeps flaking at bay without overdrying the scalp.
Factors That Make Dandruff Worse
Anything that increases sebum production or shifts your scalp’s microbial balance can worsen dandruff. Hormonal fluctuations are a big one: puberty, stress, and conditions that elevate androgen or cortisol levels all ramp up oil production. Seasonal changes matter too. Many people notice dandruff flares in winter, when cold, dry air outside and heated air indoors strip moisture from the scalp, weakening the skin barrier and making it more reactive to fungal byproducts.
Infrequent washing allows sebum and fungal metabolites to build up, creating ideal conditions for irritation. On the other hand, overly harsh shampoos can strip protective oils and compromise the skin barrier, which can also backfire. The goal is to find a washing frequency that keeps oil levels moderate without damaging the scalp’s natural defenses.