The effects fade when boys stop using the products, note the researchers.
"This report raises an issue of concern, since lavender and tea tree oil are sold over the counter in their 'pure' form and are present in an increasing number of commercial products, including shampoos, hair gels, soaps, and body lotions," write researchers Derek Henley, PhD, and others.
Henley works at the National Institute of Environmental Health Sciences (NIEHS), which is part of the National Institutes of Health (NIH).
"We want to encourage doctors who may be seeing patients with gynecomastia to ask their patients about the products they are using," says Henley's colleague, Kenneth Korach, PhD, in a NIEHS news release.
"Patients with prepubertal gynecomastia may want to consider reducing the use of products that contain these oils," says Korach.
3 Reported Cases of Gynecomastia
Henley and colleagues investigated reports of three healthy boys who hadn't started puberty and who developed breasts, a condition called gynecomastia, after using products containing lavender or tea tree oils.
It's rare for prepubertal boys to develop breasts, the researchers note.
One of the boys was 4 years old. His mother had applied "healing balm" containing lavender oil to his skin shortly before the boy's breasts developed and grew to about an inch in diameter.
Another boy was 10 and had been using a hair-styling gel daily that contained lavender and tea tree oils.
The third boy -- nearly 8 -- had been using lavender-scented soap and skin lotions. His fraternal twin had used the same products but didn't develop breasts.
The boys' breasts receded within months after they stopped using the products.
Henley's team conducted lab tests on human breast cells exposed to lavender and tea tree oils.
The tests showed that lavender and tea tree oils may boost estrogen (a sex hormone that promotes female characteristics and is linked to breast development) and hamper androgens (sex hormones that promote male characteristics and inhibit breast growth).
The researchers call for more observational studies to track prepubertal gynecomastia in boys using such products.
Lavender, Tea Tree Oils
Henley and colleagues don't mention specific products in their report, and they're not ruling out the possibility that the boys' breast growth may have stemmed from other causes.
It's often hard to pinpoint the exact cause of gynecomastia, the researchers note.
However, "We conclude that repeated topical exposure to lavender and tea tree oils probably caused prepubertal gynecomastia in these boys," write Henley and colleagues.
"We do not anticipate any long-term effects on hormone levels" in the boys, Henley says in an NIEHS news release.
SOURCES: Henley, D. The New England Journal of Medicine, Feb. 1, 2007; vol 356: pp 479-485. News release, National Institute of Environmental Health Sciences, National Institutes of Health.
By Miranda Hitti
Reviewed by Louise Chang