Lavender oil does not mimic estrogen nor does it enhance the body’s own estrogens. It is therefore not a ‘hormone disruptor’, cannot cause breast growth in young boys (or girls of any age), and is safe to use by anyone at risk for estrogen-dependent cancer. The lack of estrogenic action is the conclusion of a new report, which used a novel form of ‘uterotrophic’ assay.
This measures the effect of a test substance on the uterus of immature or estrogen-deprived female rats over three days. Any estrogenic action causes a rapid and measurable increase in uterine weight. The assay has been in use since the 1930s, was adopted by the OECD in 2007, and is now regarded as the “benchmark animal assay for estrogenic effects” (Politano 2013).
A 2007 report by Henley et al found that both lavender and tea tree oils had a weak in vitro estrogenic action. Lavender was suggested as the cause of three cases of prepubertal gynecomastia (breast growth) in boys, and tea tree was suggested in one case. The report was subsequently criticized on a number of grounds. For example it was pointed out that there was no evidence of either essential oil causing the gynecomastia in any of the four cases, and that in vitro estrogenic findings frequently do not extrapolate to a similar action in warm bodies (see Rebuttals, below).
Since 2007 there have been many warnings about lavender and tea tree oils that relate back to this study. Others, including myself, have suggested that such cautions are unnecessary and premature. The new research, in which considerable quantities of lavender oil were used, found that there was no increase in uterine weight, and so no estrogenic action.
The novel aspect of the uterotrophic assay was that the test substance – lavender oil – was applied to the skin, while the most common method is subcutaneous injection. This was changed in order to mimic the circumstances of the Henley et al 2007 case reports, and it also mimics the use of lavender oil in fragrances and personal care products. Lavender oil was used in two concentrations, 4% and 20% in corn oil. According to Politano et al 2013, these concentrations are respectively more than 6,000 and 30,000 times greater than a conservative estimate of human skin exposure from multiple cosmetic products containing lavender oil. They are also 5,000 and 1,000,000 times greater than the estimated exposure to lavender oil experienced by the Henley et al boys.
These calculations may seem exaggerated, but the massive differences are because in the uterotrophic assay, the lavender oil was applied in Hilltop Chambers patches, which do not allow any evaporation, or loss of essential oil by means other than dermal absorption. Examples of the quantities of fragrant substance absorbed from personal care products are shown below. The amount of fragrance absorbed from a shampoo, for instance, is 200 times less than the amount applied to the head. If amounts of dermally-absorbed lavender oil that are at least 5,000 times greater than any normal human exposure are not estrogenic, then we can be confident that this particular safety issue is not a concern.