Tea tree oil
Tea tree oil, or melaleuca oil, is an essential oil with a fresh camphoraceous odor and a color that ranges from pale yellow to nearly colorless and clear. It is taken from the leaves of the Melaleuca alternifolia, which is native to Southeast Queensland and the Northeast coast of New South Wales, Australia. Tea tree oil should not be confused with tea oil, the sweet seasoning and cooking oil from pressed seeds of the tea plant Camellia sinensis (beverage tea) or the tea oil plant Camellia oleifera.
Tea tree oil is toxic when taken by mouth, but is widely used in low concentrations in cosmetics and skin washes. Tea tree oil has been claimed to be useful for treating a wide variety of medical conditions. Although it shows some promise as an antimicrobial, there is insufficient evidence of its effectiveness for many of these claimed uses. Its use as a treatment for head lice in children has been recommended against.
History and extraction
The commercial tea tree oil industry originated in the 1920s when Arthur Penfold, an Australian, investigated the business potential of a number of native extracted oils; he reported that tea tree oil had promise as it exhibited powerful antiseptic properties.
Composition and characteristics
In vitro studies show that tea tree oil is capable of killing MRSA in a laboratory setting. Studies have shown that it demonstrated similar rates of eradication when compared to treatment with mupirocin. A 2005 review stated that there is insufficient evidence to recommend routine use for this purpose in a clinical setting. An 2008 article from the American Cancer Society says that studies have found some promise of a possible role for the topical application of tea tree oil as an antiseptic, but that "despite years of use, available clinical evidence does not support the effectiveness of tea tree oil for treating skin problems and infections in humans". A 2012 review by the NIH rates Tea tree oil as "possibly effective" for three applications, saying that "a 5% tea tree oil gel appears to be as effective as 5% benzoyl peroxide" for treating mild to moderate acne and that "a 10% tea tree oil cream works about as well as tolnaftate 1% cream" in treating symptoms of athlete's foot, although being less effective than clotrimazole or terbinafine.
Despite claims from proponents, there is insufficient evidence for it to be rated for other applications, including treatment of lice, cold sores, eczema, yeast infections and ringworm. There is no evidence it helps the immune system and it is poisonous when taken internally. A 2012 review of head lice treatment recommended against the use of tea tree oil for children because it could cause skin irritation or allergic reactions, because of contraindications, and because of a lack of knowledge about the oil's safety and effectiveness.
According to the American Cancer Society (2008): "Tea tree oil is toxic when swallowed. It has been reported to cause drowsiness, confusion, hallucinations, coma, unsteadiness, weakness, vomiting, diarrhea, stomach upset, blood cell abnormalities, and severe rashes. It should be kept away from pets and children." The National Capital Poison Center say tea tree oil "is poisonous if swallowed and so should not be used in or around the mouth at all".. There is at least one case of poisoning reported in medical literature. 
A 2008 EU scientific committee report into the safety of tea tree oil stated in its conclusion that some oil constituents may be absorbed by the skin, leading to "considerable systemic exposure". They further state that the oil is a skin sensitizer, and that "neat tea tree oil and certain formulations at concentrations of 5% or more can induce skin and eye irritation".
Some people can experience allergic contact dermatitis as a reaction to dermal contact with tea tree oil. Allergic reactions may be due to the various oxidation products that are formed by exposure of the oil to light and/or air.
The National Pediculosis Association say that tea tree oil is "touted as safe and natural", but that it "must be treated with respect". They say it is unsuitable for use by pregnant women and children and that it can cause allergic reactions and skin irritation.
In vitro testing of tea tree oil shows that it contains chemicals which are weakly estrogenic causing particular concern for use with children. However in tests, the chemicals which show this effect failed to show absorption into the skin, and evidence of a hormonal effect is therefore considered implausible by an EU scientific committee.
In dogs and cats, death  or transient signs of toxicity (lasting 2 to 3 days), such as depression, weakness, incoordination and muscle tremors, have been reported after external application at high doses.
Undiluted tea tree oil can cause some hearing loss when used in the ears of non-human animals; however, a 2% concentration has not been shown to have any lasting effect. It is not known whether the same is true for humans.
- Cajuput oil – derived from Melaleuca leucadendra
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