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Dandruff

Looks like: Snowflakes on your shoulders or a less poetic accumulation of yellowish scales on your scalp. Scalp can also be red and irritated.
Caused by: Oily skin getting cozy with a yeast called malessizia.
Feels: Itchy (and embarrassing).
Where: Usually the scalp but sometimes on eyebrows, eyelids, behind the ears, or in skin folds.
Contagious: No.
Treatment: Over-the-counter medicated dandruff shampoos with active ingredients such as salicylic acid, coal tar, zinc, resorcin, ketoconazole, or selenium. Shampoo daily, using fingertips to massage scalp and loosen scales.
See a doctor if: It doesn’t respond. Your doctor can prescribe stronger shampoos and medicated lotions containing selenium, ketoconazole, or corticosteroids.

Next: Athlete's foot