A recent study reignited fear and confusion in parents about one of the most versatile and trusted essential oils—lavender—and whether it has estrogenic properties. The study published in the Journal of Clinical Endocrinology & Metabolism claims an association between lavender products and premature thelarche (PT; the development of small breasts typically before age three) and prepubertal gynecomastia (PG; the growth of breast tissue in males).
Dozens of sensationalized articles ran with this study’s assertions blurring the line between good scientific journalism and clickbait publicity. Most journalists only read the study abstract and do not dive deeper into the methodology and other important details, which leads to an oversimplification and sometimes misinterpretation of the results. Many health professionals and well-meaning people spammed their family and friends with these oversimplified and misconstrued articles to “prove” they should not use essential oils. So, is it time to throw out lavender and tea tree essential oils and avoid their use in children, or is this a case of scientists untrained in and ignorant of essential oils making absurd conclusions?
Previous research findings
Previously, a study argued that chronic lavender exposure caused PG, while another study alleged that both tea tree and lavender “probably caused” PG in boys. These studies spread like wildfire because they create doubt in people’s minds whether they should use one of the most popular and effective natural solutions available—essential oils. However, one should be cautious in jumping to conclusions and remember correlation does not mean causation.
The first study contains three case reports of PG that resolved after the discontinuance of lavender-scented products. One was a shampoo with lavender listed as an ingredient and the other two regularly used the cologne agua de violetas. They analyzed the cologne with HPLC, and based upon an “assumption” and comparison with an analysis of lavender essential oil concluded that linalool and linalyl acetate were components of the cologne.
The scientists failed to provide the full HPLC analysis of the oil for verification of lavender biomarkers and to rule out synthetic and adulterant markers. Given the substantial adulteration of lavender essential oil and the common practice of perfumers to use petroleum-derived synthetic chemicals (like synthetic linalyl acetate), it is highly likely that the cologne does not contain true 
lavender essential oil but synthetic bioidentical chemicals. Synthetic adulterants could be the underlying cause of PG but further research is necessary to determine if this is the case. It is also probable that an adulterated lavender was used in the shampoo product. In addition, reports show that about half of tea tree oil samples don’t meet the composition standards to be considered genuine and pure tea tree oil.
If you want to read about the flawed science of the latter study, I invite you to review page 24 of my book Medicinal Essential Oils. For additional insight into how these studies are flawed—including confounding false-positive results due to the use of 96-well plates and the fact that essential oils may extract plastic chemicals up to 10,000-fold—see my Facebook live video.
Estrogenic activity of lavender and tea tree essential oils
Each of these studies validity is entirely dependent on lavender (or tea tree) producing sufficient estrogenic effects to disrupt the endocrine system. However, compelling evidence suggests lavender and tea tree do not have estrogenic potential.
The rat uterotrophic assay is the gold-standard test in living organisms to detect the estrogenic activity of a substance. During this test, immature female rats are repeatedly exposed to doses of a substance for three days. The weight of their uterus is measured on the fourth day, which provides a sensitive and toxicologically relevant measurement of the estrogenic activity of the tested substance.
In 2013, scientists evaluated the estrogenic effects of topical lavender in the rat uterotrophic assay. Immature female rats were administered 20 or 100 mg/kg of body weight over (roughly the equivalent of an adult applying 1.5 mL to 7.5 mL of lavender essential oil each day for three days) the course of three days. A positive control group was administered 2.5 mcg/kg of 17α-ethinyl estradiol (a synthetic estrogen used in birth control pills). The weight of the uterus and ovaries were recorded 24 hours after the final dose was administered. As expected, the positive control group that received 17α-ethinyl estradiol experienced significant increases in mean absolute and relative uterine weights. However, uterus weights of the lavender treated group did not increase relative to normal values. The scientists concluded that lavender essential oil “was not active in the rat uterotrophic assay and gave no evidence of estrogenic activity.”
Tea tree has not been tested in the uterotrophic assay, nor could any convincing research be found supporting an estrogenic or hormone-disrupting effect. Although a group of scientists contended that individual chemicals within tea tree oil disrupt hormone function, they relied upon flawed methodology that in essence makes their finding invalid (see the Facebook video above for more details).
The most recent study again used an in vitro (laboratory) method by placing human cells in 6-well, 24-well, and 48-well plates. While the authors took measures—also assessed soybean oil—to assess whether essential oils dissolved plastic chemicals in the plates to elicit the observed hormonal activity, they did not use the gold standard in vivo (performed or taking place in a living organism) rat uterotrophic assay. In vitro studies don’t always correlate with in vivo studies, and particularly human clinical studies. With the conflicting results in these studies, the appropriate approach is to weigh the in vivo study results more heavily. In other words, the in vivo study that suggests lavender has no estrogenic activity supplants the most recent study that reported lavender has estrogenic activity in laboratory research.
Lavender and tea tree essential oil major constituents
Essential oils are complex mixtures of volatile aromatic molecules. They can contain from a dozen to hundreds of constituents depending on the oil. The ratio of these molecules is what gives essential oils their therapeutic benefits. It is also known that certain essential oil constituents can synergize, buffer, or counteract the effects of another constituent in the oil.
To aid in this discussion the typical percentages of major constituents in lavender and tea tree essential oil are provided.
Lavender
- Linalyl Acetate 25%–45%
- Linalool 25%–45%
- (Z)-Beta-Ocimene 2%–8%
- Beta-Caryophyllene 2%–6%
- Lavandulyl Acetate 2%–6%
- Terpinen-4-ol 1%–6%
Tea Tree Oil
- Terpinen-4-ol 35%–48%
- Gamma-Terpinene 10%–28%
- Alpha-Terpinene 5%–13%
- Alpha-Terpineol 2%–8%
- Alpha-Pinene 1%–6%
- Terpinolene 1%–5%
Penetration enhancing effects of essential oil constituents
Essential oils have been used to enhance the penetration and absorption of various substances through the skin. Indeed, scientists have observed this effect of essential oils for decades in relation to the transdermal delivery of drugs. Knowing this, synthetic chemicals—some of which are known hormone disruptors—are likely to penetrate through the skin at a higher rate when combined with essential oils.
Terpenes are the most common class of constituent found within essential oils and they are known to acts as penetration enhancers of multiple substances (vitamins, drugs, phytocompounds, etc.). Some of these individual terpenes have been explored for their ability to act as penetration enhancers. It is also possible that essential oils that contain multiple penetration-enhancing constituents—like tea tree and lavender—could possess a synergistic effect.
Lavender essential oil itself is a known penetration enhancer. Looking at the major components in lavender we find that the following are known penetration enhancers:
Tea tree essential oil also contains constituents that enhance the penetration of other ingredients or chemicals through the skin:
- Tea tree oil (1)
- Terpinen-4-ol (2),(3)
- Alpha-terpineol (4),(5),(6)
- Alpha-pinene (7),(8)
- Alpha-terpinene (9)
These lists are far from inclusive of all constituents in these two oils that increase the penetration of other substances through the skin.
Endocrine-disrupting chemicals in the products
A more plausible answer for the link between products that allegedly contain essential oils, isolated essential oil constituents, or synthetic fragrances and PG or PT is interactions with other known hormone-disrupting chemicals present in the products. Many commercial personal care products (shampoo contains parabens and phthalates, which disrupt hormone activity) and colognes (many contain phthalates that disrupt hormone function) are a smorgasbord of harmful chemicals. Not to mention that essential oils may react with the chemicals (like bisphenol A, a known hormone disruptor) in the plastic containers of these products and cause them to leach into the product. Application of the product to the skin leads to absorption of both the plastic container chemicals and the synthetic chemical ingredients in the product.
While ingredients in wash-off products like shampoo and soaps have minimal contact with the skin, repeated exposure to these chemicals can have a cumulative effect. This is particularly true if the chemicals are being absorbed at a higher rate because the product includes lavender or tea tree essential oil. Colognes and perfumes are often sprayed directly on the skin and can have a higher absorption rate as a leave-on product.
In reality, exposure to hormone-disrupting chemicals is pervasive today because they are found in our water, air, clothes, food, furniture, vehicles, household cleaners, industrial chemicals, cosmetics, and personal care products. Indeed, one study found that infants are born with a tremendous toxic burden—an average of 200 chemicals present in umbilical cord blood at birth. Exposure to additional hormone-disrupting chemicals that are absorbed at a higher rate due to the presence of essential oils may have simply been the tipping point to trigger PG or PT.
Cross sectional study searching for endocrine system disruption
Further poking holes in the oft-repeated misinformation about lavender and tea tree oils disrupting endocrine system function, in 2022 a group of researchers performed a cross sectional study to compare the prevalence of endocrine disruption in children aged 2 to 15 years old. Of the 556 children included in the study, only nine children experienced an endocrine disruption-related disorder (two cases of precocious puberty, one case of delayed puberty, four cases of hypothyroidism, and one case of growth hormone deficiency). The number of these cases among the study population was consistent with population norms, meaning even if they were never exposed to essential oils, you’d expect to see this many cases of endocrine related disorders. Zero cases of PG were observed. Analysis of the participants to determine if endocrine disorders were more prevalent among children with routine exposure to essential oils when compared to those not exposed to essential oils, demonstrated no increased risk among the children regularly exposed to essential oils. Indeed the average lifetime prevalence of PG in the mass population is 2.5% and this study showed a prevalence of 0% among children regularly exposed to essential oils. In a nutshell, the study concluded that “children who were regularly exposed to lavender or tea tree essential oils experienced the same risk of endocrine disorders as those who were not exposed.”
The evidence-based and logical conclusion
Dozens of clinical trials using lavender or tea tree, combined with anecdotal use among millions of children, has not reported a statistically significant amount of estrogenic activity or hormone disruption for these two oils. We are each biologically unique and will respond to different substances uniquely. Most people can eat watermelon, but some have their throat itch and close if they do. Some children can eat peanuts, but to others, peanuts represent a threat to their life. Even among natural products, you will have a very small number of people that respond adversely or experience undesirable effects.
The presumed link to essential oils and hormone disruption in these studies is far from proven and frequently based on poor science and flawed methodology. The observations do not prove definitively PT or PG was caused by essential oils, especially since the products are fragranced and likely don’t contain any pure lavender or tea tree oil. This association has been grossly overstated and aggrandized by the media. What we can learn from these studies is that consumers should be more selective about their products. They should choose natural products and products with pure essential oils that do not contain hormone-disrupting chemicals. Unfortunately, this is a major trend among companies selling soaps, lotions, shampoos, and other personal care products to capitalize on the growing essential oil industry by adding essential oils (pure or not) with other fragrance ingredients.
The bottom line is you can still safely use tea tree and lavender essential oils on your children when diluted appropriately. With anything in medicine or health, the risks versus the benefits have to be weighed. And in this case, the vast whole-body benefits of these two essential oils far outweigh the minimal and rare risks that are far from proven by any reasonable and credible science.