Reports are again circulating on the internet that lavender essential oil and tea tree essential oil mimic estrogen and are hormone disrupters, specifically, that those essential oils disrupt male hormones and cause gynecomastia (breast growth) in boys. I first heard this years ago and researched it then, but now with the reports recirculating, it’s worth sharing what I’ve learned so you can make your own informed decision.
The Claim: All the claims on the internet lead back to one article to have made this alleged correlation: a "Brief Report" in the New England Journal of Medicine from February 2007 entitled "Prepubertal Gynecomastia Linked to Lavender and Tea Tree Oils" (Henley, et al). The Brief Report documented three cases of gynecomastia in boys aged four to seven who had been using unspecified products which were self-reported to have contained lavender "and/or" tea tree essential oils. Once the boys stopped using the products, the breasts went away.
Four things with the Brief Report jumped out at me: (1) the Report did not name the products suspected as being responsible for the effects in question, meaning that the products could have contained other endocrine disrupting ingredients to blame for the gynecomastia, including compounds leached from the likely BPA-containing plastic containers; (2) the Report did not present any information about the concentration, combination, or purity of the lavender "and/or" tea tree essential oils in any of the products used; (3) the products the boys were using were self-reported to have contained the essential oils; and (4) the Report only involved three subjects, a terribly small sample size insufficient to generalize for the entire population, in an uncontrolled study.
Numerous rebuttals to the Brief Report from the medical community were sent to the New England Journal of Medicine, including one by three doctors representing Wake Forest, Yale and Harvard (Kemper, Romm, and Gardiner, respectively) who commented:
The study by Henley et al. (Feb. 1 issue) raises many questions. Product names were not provided. Did the authors contact manufacturers to report concerns or ask about constituents? The variability, adulteration, and contamination of herbal products have been widely reported, as have discrepancies between labels and contents. Plastic containers may contain phthalates, known endocrine disrupters. What was actually in the products cited in this report?
None of the hormonal testing showed abnormal results, except in Patient 2, who had elevated levels of testosterone (not estrogen). There was no report on ultrasound examination or needle biopsy, nor were subsequent weight changes reported. Might the patients' gynecomastia have reflected another pathophysiological process that resolved spontaneously?
Traditional use and clinical trials have not suggested estrogenic effects of tea tree or lavender oil, though estrogenic effects have been reported for other essential oils and plants. Are occupational exposures to lavender and tea tree associated with estrogenic symptoms? In vitro testing alone is not adequate grounds for indicting traditionally used products and may raise public fear.
As Kemper, Romm, and Gardiner point out, the authors of the Report conducted in vitro testing (testing carried out in an artificial environment, such as a test tube or a petri dish). They performed a test wherein the essential oils were applied to human cells in a petri dish. The Report claimed that the results of the test indicated that lavender and tea tree essential oils have "weak" estrogenic and anti-adrogenic effects. However, it should be noted that the authors of the Report did not apply pure essential oils to the human cells in the test; rather, they diluted the oils with a solvent, dimethylsulfoxide, which itself is an estrogen mimicker. Moreover, the plastic labware used in the tests contained estrogenic compounds, such as phthalates, which can be leached from the plastic by the solvent properties of essential oils and may have led to estrogenic activity being mistakenly ascribed to lavender and tea tree essential oils.
Another doctor's rebuttal (Kalyan) to the Brief Report noted about the in vitro testing:
Although Henley et al. attempt to show that these oils have estrogenic activity, the results of their reported assays indicate a very weak effect. It would be bewildering if such relatively low hormonal activity alone could instigate prepubertal gynecomastia.
Two of the authors of the Brief Report (Korach and Bloch) responded to the rebuttals, agreeing that their findings should be interpreted carefully, admitting to the possibility that the estrogenic effects could be modified by other disrupters, accepting that there may be other essential oils that could have contributed to the clinical findings in their subjects, and agreeing that further scientific studies were necessary.
Further Study: The results of one such further scientific study was published in the March/April 2013 volume of the International Journal of Toxicology. This study, titled "Uterotrophic Assay of Percutaneous Lavender Oil in Immature Female Rats" (Politano, et al), evaluated the estrogenic potential of lavender oil using a "uterotrophic assay" -- an in vivo test, meaning it was carried out in a living organism. A "uterotropic assay" measures the effect of a test substance -- here, lavender essential oil applied topically to mimic the circumstances of the Henley et al Brief Report of 2007 and to mimic the use of lavender oil in personal care products -- on the uterus of immature or estrogen-deprived female rats over three days. In the assay, lavender essential oil was used in concentrations more than 6,000 and 30,000 times greater than a conservative estimate of human skin exposure from multiple cosmetic products containing lavender oil. The concentrations were also 5,000 and 1,000,000 times greater than the estimated exposure to lavender essential oil experienced by the boys in the Brief Report of 2007. The 2013 study concluded that, even at these high concentrations, lavender essential oil gave no evidence of estrogenic activity.
Bottom Line: Of course, this single test cannot be taken as conclusive, just as the single Brief Report which started this discussion cannot be taken as conclusive, but the bottom line is that the one report (that is, the Henley et al Brief Report) that has become the "authority" for linking lavender and tea tree essential oils to hormone disruption is poorly constructed, inconclusive, and mostly anecdotal. The one study of the estrogenic potential of lavender essential oil that we do have (that is, the Politano et al 2013 study), which used an uterotropic assay considered the “benchmark" assay for estrogenic effects, concludes that lavender essential oil is not estrogenic. In short, at this time, concerns are unnecessary and premature.
I hope this has given you some information to make an informed decision of your own. I look forward to seeing more peer-reviewed studies on the subject.