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Antimicrobials

Tea tree oil

Also listed as Melaleuca alternifolia leaf oil, melaleuca oil

An antimicrobial essential oil. 5% gels helped mild acne and a 5% shampoo helped dandruff in trials, but it's a common allergen, especially once oxidized.

Limited evidenceTypical strength: 5% in the positive trials

What it is

Tea tree oil is distilled from the leaves of an Australian tree. Its main component, terpinen-4-ol, kills bacteria and yeasts in the lab, including the acne bacterium and the yeast linked to dandruff. It turns up in acne gels, shampoos, body washes and some toothpastes.

The trials used 5%; many products contain far less or don't say. Neat oil irritates skin and is toxic if swallowed.

How strong is the evidence?

Limited evidence

Small randomized trials support 5% tea tree oil for mild to moderate acne (better than placebo; slower than 5% benzoyl peroxide but with fewer side effects) and for dandruff (41% vs 11% improvement with placebo over 4 weeks). Trials are small and short, and contact allergy is well documented.

What it's used for, checked

Mild acne

Holds up

5% gel beat placebo in one trial and worked more slowly but more gently than 5% benzoyl peroxide in another.

Dandruff

Holds up

A 5% shampoo improved dandruff severity 41% vs 11% with placebo over 4 weeks.

Fresher breath and healthier gums (toothpaste)

Unproven

Too little oil and too few trials to tell.

Who should be careful

  • A common cause of allergic contact dermatitis; oxidized (old, air-exposed) oil is more allergenic, so keep bottles capped and discard old oil.
  • Never apply undiluted oil or swallow it; ingestion has caused serious poisoning, especially in children.
  • Case reports linked repeated use of lavender and tea tree products to breast growth in young boys that reversed after stopping; causation is debated, but limiting use in prepubertal children is a cautious choice.

The studies

Every citation is checked against its PubMed record.

  1. A comparative study of tea-tree oil versus benzoylperoxide in the treatment of acne.
    Bassett IB, Pannowitz DL, Barnetson RS · Medical Journal of Australia · 1990 · RCT (single-blind)

    124 patients with mild to moderate acne used 5% tea tree oil gel or 5% benzoyl peroxide lotion; both reduced inflamed and non-inflamed lesions, tea tree oil more slowly but with fewer side effects.

  2. The efficacy of 5% topical tea tree oil gel in mild to moderate acne vulgaris: a randomized, double-blind placebo-controlled study.
    Enshaieh S, Jooya A, Siadat AH, Iraji F · Indian Journal of Dermatology, Venereology and Leprology · 2007 · RCT

    60 patients with mild to moderate acne used 5% tea tree oil gel or placebo for 45 days; tea tree oil was 3.55 times more effective on total lesion count and 5.75 times on severity, with similar, tolerable side effects.

  3. Treatment of dandruff with 5% tea tree oil shampoo.
    Satchell AC, Saurajen A, Bell C, Barnetson RS · Journal of the American Academy of Dermatology · 2002 · RCT (single-blind)

    126 people with mild to moderate dandruff used 5% tea tree oil shampoo or placebo daily for 4 weeks; severity scores improved 41% versus 11%, with no adverse effects.

  4. Prepubertal gynecomastia linked to lavender and tea tree oils.
    Henley DV, Lipson N, Korach KS, Bloch CA · New England Journal of Medicine · 2007 · Case series + laboratory study

    Three prepubertal boys developed breast enlargement while using products containing lavender and tea tree oils; it resolved after they stopped, and both oils showed weak estrogenic and anti-androgenic activity in human cell lines. Causation was disputed in later correspondence.

  5. Tea tree oil: contact allergy and chemical composition.
    de Groot AC, Schmidt E · Contact Dermatitis · 2016 · Review

    Tea tree oil has caused more published allergic reactions than any other essential oil; 0.1-3.5% of people patch-tested react. Fresh oil is a weak-to-moderate sensitiser, but oxidised (old, air-exposed) oil is much stronger. Most cases come from neat oil, a minority from cosmetics.