Seborrheic dermatitis

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In short

Dandruff is usually a symptom of seborrheic dermatitis, or seborrhea. It is a skin condition that can also cause redness and irritation of the skin.

What it is1

Seborrheic dermatitis is a long-lasting condition that mostly affects oily areas such as the scalp, face and upper chest. It causes poorly defined red patches with greasy or yellowish scales.
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Itching is the most common symptom.
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It is chronic and recurrent, which means it tends to come back after it clears.
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Dandruff is not caused by poor hygiene, although it may be more obvious if you do not wash your hair regularly.
NHS website
It may be caused by different things, including a type of yeast that naturally lives on your skin, and skin conditions such as eczema.
NHS website
Stress and cold weather may also make dandruff worse.
NHS website

Use an anti-dandruff shampoo to treat dandruff.
NHS website
Look for a shampoo that contains 1, or more, of these ingredients: ketoconazole; coal tar; salicylic acid.
NHS website
Use the shampoo for up to 4 weeks to see if your dandruff gets better.
NHS website
Most of the time, using a dandruff shampoo can help control your dandruff.
MedlinePlus

See a health care provider if you still have dandruff symptoms after using anti-dandruff shampoo for at least 4 weeks; your dandruff is bad or your scalp is very itchy; your scalp is red or swollen; you have flaky, itchy patches on your face or other parts of your body.
NHS website

These pages describe what the sources say. They cannot say what is going on with your skin.

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