Why does seborrheic dermatitis happen?

How do yeast and skin oil cause a rash?

Malassezia uses sebum, the oil your skin makes, which is a remarkably local catering arrangement. The yeast produces enzymes that break down fats in sebum, releasing fatty acids that can trigger inflammation. Inflammation changes how the outer skin cells develop and shed, producing scale and weakening the skin barrier, the outer layer that helps keep water in. That disruption lets water escape more easily and gives the yeast greater access to the outer skin layers.34

This helps explain why seborrheic dermatitis favours the scalp, eyebrows, sides of the nose, ears and chest, where oil glands are plentiful. But oily skin alone does not explain it: people with seborrheic dermatitis can have normal oil production, and people with very oily skin can have no dermatitis. The makeup of skin oils and the skin's response to yeast also appear to matter. A higher yeast count is not a consistent finding in affected skin.23

Is it caused by poor hygiene or an allergy?

Seborrheic dermatitis is not caused by unclean skin and is not an allergy. Everyone has yeast on their skin; only some people develop this reaction. Finding Malassezia in a skin scraping does not diagnose seborrheic dermatitis, because the yeast belongs on normal skin too.13

Why does it flare up?

Triggers worsen an existing tendency rather than explain the whole cause. Stress, fatigue, lack of sleep and cold, dry weather are linked to flares. Alcohol based lotions and infrequent shampooing can aggravate symptoms, even though unclean skin is not the cause. Many people improve in warm, humid weather. In adults, seborrheic dermatitis commonly comes and goes, so a returning rash does not necessarily mean the treatment failed.1235

Who is more likely to develop it?

Many people with seborrheic dermatitis are otherwise healthy. Risk is higher with certain conditions, including Parkinson's disease, HIV, depression and immune suppression after an organ transplant. A family tendency is also associated with the condition. Some oral prescription medicines, including lithium and haloperidol, are linked to its onset or flares. If a rash starts after a new medicine, contact the prescriber rather than stopping or changing the medicine yourself.135

Dandruff is a scalp form with flaking and little or no visible inflammation. Babies can develop cradle cap, which usually clears within weeks to months; adult seborrheic dermatitis often needs ongoing control.346

What should you do about it?

Treatment targets yeast, inflammation and scale rather than simply removing oil. Different dandruff shampoos do different jobs, despite sharing a shelf as though that settled the matter, which is a lot of authority to give a shelf. Some control yeast; others reduce scale, which makes 'dandruff shampoo' a category rather than a job description. For scalp symptoms, use the chosen shampoo as directed and leave it on for the instructed contact time. Frequency depends on the shampoo and hair type, and weekly maintenance is often used after symptoms settle.47

For affected facial or body skin, gentle cleansing and a prescribed antifungal cream are common approaches; courses often last 2 to 4 weeks. A clinician can add a brief anti inflammatory treatment if needed. Avoid scrubbing. Babies need a separate approach with baby shampoo and gentle removal of loosened scales, not adult dandruff shampoo.357

Get the rash assessed if home treatment does not help or symptoms worsen. Psoriasis and other rashes can look similar or occur alongside seborrheic dermatitis. Seek medical care for fever, marked tenderness, swelling or streaks of redness, which can indicate infection.57