The oil contains numerous chemicals known as terpenoids. Australian standards were established for the amount of one particular compound, terpinen-4-ol, which must make up at least 30% and preferably 40–50% of the oil for it to be medically useful. Another compound, cineole, should make up less than 15% and preferably 2.5% of the oil. The oil kills fungus and bacteria, including those resistant to some antibiotics. For common acne, a double-blind trial compared the topical use of 5% tea tree oil to 5% benzoyl peroxide. Although the tea tree oil was slower and less potent in its action, it had far fewer side effects and was thus considered more effective overall.
A double-blind trial found that a 10% tea tree oil cream was as effective as anti-fungal medicine at improving symptoms associated with athlete’s foot, though it was not more effective than a placebo for eliminating the fungal infection. A double-blind trial found 100% tea tree oil applied topically was as effective as the anti-fungal medicine clotrimazole (Lotrimin®, Mycelex®) for people with fungus affecting the toe nails, a condition known as onychomycosis. In another double-blind trial with toenail fungus sufferers, a combination of 5% tea tree oil and 2% butenafine (Mentax®), a synthetic anti-fungal drug, in a cream proved more effective than an unspecified concentration of tea tree oil in cream alone. The results are not entirely surprising, as the tea tree product alone was probably not at a sufficiently high enough concentration to be effective.
A preliminary trial found that rinsing the mouth with 1 tablespoon (15 ml) tea tree oil solution four times daily effectively treated thrush (oral yeast infections) in AIDS patients. Solutions containing no more than 5% should be used orally and should never be swallowed.
A concern for hospital staff and patients is the spread of the bacteria Staphylococcus aureus—sometimes referred to as a “staph infection.” One small clinical trial found that use of a 4% tea tree oil nasal ointment as well as a 5% tea tree oil body wash was slightly more effective than standard drugs used to prevent the spread of the bacteria.