Caspa (dandruff) happens because of a fungus that already lives on your scalp. A yeast called Malassezia feeds on your skin’s natural oils and leaves behind irritating byproducts that cause flaking, itching, and sometimes redness. Nearly everyone has this fungus on their skin, but some people’s scalps react to it more strongly than others.
The Fungus Behind the Flakes
Malassezia can’t produce its own fatty acids, so it survives by breaking down sebum, the oily substance your scalp naturally secretes. It uses enzymes called lipases to decompose the fats in sebum into saturated and unsaturated fatty acids. The fungus consumes the saturated fatty acids for energy and leaves the unsaturated ones behind, especially oleic acid.
That leftover oleic acid is the real problem. It accumulates on the outermost layer of your skin and disrupts the skin barrier, increasing water loss and triggering inflammation. In people who are susceptible, this irritation speeds up skin cell turnover. Your scalp sheds roughly 487,000 cells per square centimeter under normal conditions. During active dandruff, that number climbs to around 800,000 cells per square centimeter. Those excess dead cells clump together with oil and fall off as visible white or yellowish flakes.
Not everyone reacts the same way to oleic acid. Some people can have high levels of Malassezia on their scalp with no symptoms at all, while others develop flaking and itching even with modest fungal populations. This individual sensitivity is a big part of why dandruff runs in some families and not others.
What Makes It Worse
Several factors can tip the balance in the fungus’s favor or make your scalp more reactive.
Oily skin: The more sebum your scalp produces, the more food Malassezia has. Sebum production peaks during adolescence and young adulthood, which is why dandruff tends to appear after puberty and is most common in younger adults. There’s a second rise in prevalence after age 50.
Stress: Periods of high stress can increase oil production on your skin, creating conditions that favor fungal growth. Stress also weakens immune responses that normally keep Malassezia in check.
Cold, dry weather: Winter often makes things worse, though the mechanism is a bit counterintuitive. Cold outdoor air and indoor heating strip moisture from your scalp, weakening the skin barrier. A compromised barrier is more vulnerable to the irritating fatty acids Malassezia leaves behind. It’s worth noting that a dry, flaky scalp in winter isn’t always dandruff. True dandruff tends to produce larger, oilier flakes, while simple dryness causes smaller, finer flakes without the greasy feel.
Infrequent washing: Going longer between washes lets sebum accumulate on your scalp. In studies tracking scalp conditions after shampooing stopped, itch and visible flakes increased dramatically within days, accompanied by a 100- to 1,000-fold increase in Malassezia levels. Sebum buildup becomes progressively more chemically modified and irritating over time, with itch severity rising significantly within 72 hours of the last wash.
Dandruff vs. Something More Serious
Dandruff exists on a spectrum with seborrheic dermatitis, a more severe version of the same process. Mild flaking with occasional itch is typical dandruff. When you see persistent redness, crusted patches, and scaling that spreads to your eyebrows, nose creases, or behind your ears, that’s seborrheic dermatitis.
Scalp psoriasis can look similar but has key differences. Psoriasis scales tend to be thicker and drier. The patches often extend beyond the hairline onto the forehead or neck. And psoriasis usually shows up on other parts of the body too, particularly the elbows, knees, or lower back. You might also notice small pits or dents in your fingernails, which is a telltale sign of psoriasis that doesn’t occur with dandruff.
How Anti-Dandruff Shampoos Work
Most medicated shampoos attack the problem at its source by reducing Malassezia populations, slowing skin cell turnover, or both. The main active ingredients you’ll find over the counter include zinc pyrithione (typically at 1%), ketoconazole (2%), and selenium sulfide (1% or 2.5%). All of them have solid clinical evidence behind them.
In a head-to-head trial of 246 people with moderate to severe dandruff, 2.5% selenium sulfide and 2% ketoconazole both significantly outperformed placebo. Selenium sulfide showed the strongest ability to slow excess skin cell production and break down accumulated scale. One practical concern with ketoconazole: researchers have found increasing numbers of Malassezia strains resistant to it, so if a ketoconazole shampoo stops working after months of use, switching to a different active ingredient is a reasonable move.
The most common mistake people make with medicated shampoos is rinsing them out too quickly. These products need time to penetrate the scalp and work. Lather the shampoo into your scalp and leave it on for about five minutes before rinsing. Using it two to three times per week is a typical starting frequency, with regular shampoo on the other days. Once flaking is under control, you can often reduce to once a week for maintenance.
Daily Habits That Help
Washing your hair regularly is one of the simplest and most effective things you can do. The research is clear: daily washing with an anti-dandruff shampoo reduces surface oil, lowers fatty acid irritants, and keeps Malassezia populations in check. You don’t necessarily need to wash every day forever, but if you’re in a flare, increasing your wash frequency helps break the cycle faster than any other single change.
Between washes, resist the urge to scratch. Scratching inflamed skin damages the barrier further and can introduce bacteria, leading to secondary infections that make everything worse. If itching is intense, that’s usually a sign the medicated shampoo needs more time to work, or you may need a different active ingredient.
Rotating between two shampoos with different active ingredients every few weeks can help prevent the fungus from adapting. For example, alternating between a zinc pyrithione formula and a selenium sulfide formula gives you broader coverage. If over-the-counter options don’t make a noticeable difference after four to six weeks of consistent use, a dermatologist can evaluate whether you’re dealing with straightforward dandruff or a related condition that needs a different approach.