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Concern has been raised about a “possible link between gynecomastia, topical lavender, and tea tree oil.” Gynecomastia is the abnormal development of breast tissue. I talked about lavender before, but what about tea tree oil? It all started with a case series published in the New England Journal of Medicine.

They describe three young boys in whom breast growth coincided with the topical application of products that contained lavender and tea tree oils. How do we know the products were to blame? Well, the problem resolved in each patient shortly after the use of products containing these oils was stopped. Furthermore, studies in human cell lines indicated that the two oils had pro-female hormone and anti-male hormone activities. So, they “conclude that repeated topical exposure to lavender and tea tree oils probably” was the cause.

But, as a tea tree oil company representative pointed out, only one of the three boys was exposed to any amount of tea tree oil, and they were also exposed to lavender oil; so, lavender oil may have been to blame in all three cases. The researchers responded that this may be a valid argument; however, the tea tree oil had activity similar to that of lavender oil with respect to the in vitro effects. Let me walk you through those.

This is measuring the growth of human breast cancer cells in a petri dish. You drip a tiny amount of estrogen on them, and you can spike their growth, more than a dozen-fold. But, if you add an estrogen blocker along with the estrogen, it abolishes the effect. Okay.

Now, let’s add increasing amounts of tea tree oil to the breast cancer cells. Their growth goes up, and the reason we know it’s an estrogenic effect is that when you add the estrogen blocker too, the growth comes down. Pretty convincing, but “in vitro testing alone is not adequate grounds for indicting traditionally used products,” argued some herbal proponents—including Paula Gardiner, I was excited to see: one of my medical school classmates and friends—hi Paula!

Specifically, argued the Tea Tree Oil Industry Association, only a few tea tree oil compounds actually make it through the skin. So, what they should have done is just measure the hormonal effects of those three compounds. But, that had never been done…until later that year.

Yes, drip increasing concentrations of whole tea tree oil on breast cancer cells in a petri dish, and you can increase their growth, compared to an oil with no estrogenic effect, like eucalyptus oil. But, if you just look at the three components of tea tree oil that actually make it into your bloodstream when you apply them on your skin, none appears to have any estrogenic effects; so, none of the components that penetrate the skin appears to act as an estrogen—alone or in combination. So, you can’t extrapolate the petri dish effects of the whole oil to what one might see when applied on the skin. Thus, what you see in a petri dish may not be identical to what you see in a person.

These new data led European consumer safety officials to conclude that the purported link between gynecomastia and the topical use of tea tree oil is, therefore, considered implausible. In fact, if the anti-male hormone components of tea tree oil remain on the skin, well, how about using it to treat women who feel they are affected by excessive hairiness? Such women were instructed to spray themselves with a dilute lavender/tea tree oil spray versus placebo twice a day on “problem” areas (morning and evening) for three months. “[H]airs were taken “before and after” from four different body areas: chin, chest, thigh, and upper arms.” After three months, no change in the hair diameter in the placebo group, as expected. But, in the lavender/tea tree oil group, all the hairs ended up thinner.

This showed that at least the combination of “lavender and tea tree oils applied locally on skin could be effective in reducing mild [excessive hairiness],” potentially representing “a safe, economic, and practical instrument in the cure of this [quote unquote] disease.”

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