Tea tree oil has some interesting research demonstrating it to be an effective antimicrobial agent. The Journal of Applied Microbiology (January 2000, pages 170-175) stated that “The essential oil of Melaleuca alternifolia (tea tree) exhibits broad-spectrum antimicrobial activity. Its mode of action against the Gram-negative bacterium Escherichia coli AG100, the Gram-positive bacterium Staphylococcus aureus NCTC 8325, and the yeast Candida albicans has been investigated using a range of methods…. The ability of tea tree oil to disrupt the permeability barrier of cell membrane structures and the accompanying loss of chemiosmotic control is the most likely source of its lethal action at minimum inhibitory levels.” In addition, “In a randomized, placebo-controlled pilot study of tea tree oil in the treatment of herpes cold sores, tea tree oil was found to have [a] similar degree of activity as 5% acyclovir” (Source: Journal of Antimicrobial Chemotherapy, May 2001, page 450). For acne there is also some published information showing it to be effective as a topical disinfectant for killing the bacteria that can cause pimples (Source: Letters in Applied Microbiology, October 1995, pages 242-245). But there is also a medical review of tea tree oil challenging the viability of these studies (Source: Journal of Antimicrobial Chemotherapy, February 2003, pages 241-246).
However, the crux of the matter for any potential efficacy in a commercial product is, how much tea tree oil is needed to have an effect? The MedicalJournal of Australia (October 1990, pages 455-458) compared the efficacy of tea tree oil to the efficacy of benzoyl peroxide for the treatment of acne. A study of 119 patients using 5% tea tree oil in a gel base versus 5% benzoyl peroxide lotion was discussed. There were 61 patients in the benzoyl peroxide group and 58 in the tea tree oil group. The conclusion was that “both treatments were effective in reducing the number of inflamed lesions throughout the trial, with a significantly better result for benzoyl peroxide when compared to the tea tree oil. Skin oiliness was lessened significantly in the benzoyl peroxide group versus the tea tree oil group.” However, while the reduction of breakouts was greater for the benzoyl peroxide group, the side effects of dryness, stinging, and burning were also greater—“79% of the benzoyl peroxide group versus 49% of the tea tree oil group.”
Given these results, a 2.5% strength benzoyl peroxide solution would be better to start with to see if it is effective, rather than starting with the more potent and somewhat more irritating 5% or 10% concentrations. However, if you were interested in using a 5% strength tea tree oil solution to see if that would be effective, at this time I know of no products stating how much tea tree oil they contain. It appears that almost all tea tree oil products on the market contain little more than a 1% concentration, if that, which is probably not enough to be of much help for breakouts.