Tea Tree Oil
In plain terms
The antiacne essential oil. It works against acne bacteria and mites, but it is also one of the more common plant-oil allergens, especially once the bottle has aged and oxidized. Dilute it, keep it fresh, and never swallow it.
What the evidence shows
Tea tree oil is steam-distilled from the Australian Melaleuca alternifolia, and its main active, terpinen-4-ol, gives it real antimicrobial and anti-mite activity, with modest evidence for acne. The catch is that it is a recognized skin sensitizer. Fresh oil is a weak to moderate allergen, but exposure to air, light, and time oxidizes it into stronger sensitizers like ascaridole, and oxidized tea tree oil is a common enough cause of allergic contact dermatitis that dermatologists add it to standard patch-test panels. Undiluted oil also irritates skin at full strength, so it is meant to be diluted. Two more points worth stating plainly: tea tree oil is toxic if swallowed and should never be taken by mouth, and the widely repeated claim that it causes breast growth in boys traces to a single case report, which the EU's safety committee judged implausible because the oil's hormone-active components do not penetrate skin. So this is a useful antimicrobial that asks for respect: dilute it, store it sealed and away from light, replace it when it smells sharp or turpentine-like, and patch-test first.
On the label
Listed as melaleuca alternifolia leaf oil or tea tree oil. Buy small, dark-glass bottles, keep them closed and cool, and toss oil that has gone sharp-smelling, since oxidized oil is the most allergenic. Dilute before applying, and stop if you see redness or itching. If you react to fragrances or other essential oils, you are more likely to react to this one.