Mild dandruff is flakes; proper seborrhoeic dermatitis is flakes plus redness, itching and sometimes oily yellow scaling around the nose, eyebrows and behind the ears. Both are linked to an overgrowth of the yeast Malassezia and an inflammatory response on the scalp. They are common, harmless, and respond well to the right medicated shampoo — but the shelf is confusing, the ingredients do different things, and cosmetic shampoos labelled "anti-dandruff" often do very little.

Here is a straight comparison of the four active ingredients that actually have evidence, as used in UK pharmacies in 2026.

The four active ingredients

Ketoconazole 2%

Sold as Nizoral, and as cheaper generics. The strongest antifungal available without prescription for scalp use.

• Evidence: consistently best-performing in head-to-head trials for seborrhoeic dermatitis.
• Use: twice a week for four weeks, then once weekly for maintenance.
• Side effects: rare scalp irritation or changes in hair texture.
• Availability: pharmacy-only (P medicine), not general sale.

Coal tar

Tar shampoos (Polytar, Capasal, T/Gel) have decades of evidence for dandruff, scalp psoriasis and seborrhoeic dermatitis.

• Use: twice weekly, lather and leave for five minutes.
• Side effects: distinctive smell, can stain light-coloured hair.
• Note: some formulations combine tar with salicylic acid for thicker scaling.

Selenium sulphide 2.5%

Selsun and similar. Slows skin cell turnover and reduces yeast.

• Use: twice weekly for two weeks, then as needed.
• Side effects: can dry the hair and occasionally discolour blonde or dyed hair.
• Not suitable for broken skin or use near the eyes.

Zinc pyrithione

Found in most supermarket and drugstore anti-dandruff shampoos (Head and Shoulders and generics). Mild antifungal and anti-inflammatory effect.

• Use: daily or as normal shampoo.
• Best for mild dandruff, less effective for inflamed seborrhoeic dermatitis.
• Widely available, no pharmacy supervision needed.

Quick comparison

How to use a medicated shampoo properly

Most people under-use them. Four practical tips that make a real difference:

1. Apply to a dry or damp scalp, not wet — dilution reduces contact.
2. Massage in and leave on for at least five minutes. Do not rinse immediately.
3. Use twice a week for four weeks before judging whether it works.
4. When it works, keep using it once a week for maintenance. Stopping completely usually brings the dandruff back.

When to escalate

Most cases settle with medicated shampoo alone. See a pharmacist or GP if:

• There is no improvement after four weeks of correct use.
• Redness is spreading to the face, around the nose, eyebrows or chest.
• The skin is cracked, weeping, or has yellow crusts (possible infection).
• You have a weakened immune system — seborrhoeic dermatitis can be more severe and widespread in HIV and some cancers.
• You are treating a baby (different rules apply — see our cradle cap guide).

A GP may add topical steroid lotions, ciclopirox shampoo, or occasionally short courses of oral antifungals.

What does not help

• Over-washing. Daily use of harsh shampoos can make flaking worse.
• Switching products every week. Active ingredients need four weeks to show effect.
• Natural apple cider vinegar or tea tree oil as a sole treatment — limited evidence and can irritate.
• Hair oils applied heavily — they feed the yeast involved in seborrhoeic dermatitis.

Finding a pharmacy

Ketoconazole and coal tar shampoos are sold from a pharmacist's counter and come with advice on use. Find a branch with PharmSee's pharmacy directory.

Sources

• NHS: Dandruff
• NICE Clinical Knowledge Summaries: Seborrhoeic dermatitis
• British Association of Dermatologists: Seborrhoeic dermatitis patient leaflet
• British National Formulary: ketoconazole shampoo, coal tar, selenium sulphide, pyrithione zinc