NEW YORK (Reuters Health) - Three cases of breast tissue enlargement in young boys, reported in The New England Journal of Medicine, apparently resulted from the application of products containing lavender or tea tree oil.

The development of prominent breast tissue in men or adolescences, also known as "gynecomastia," is extremely uncommon in prepubertal boys, the authors note. If it does occur in a young boy, it may be due to a condition that is disrupting hormone activity.

Co-author Dr. Clifford A. Bloch, a pediatric endocrinologist in Greenwood Village, Colorado, diagnosed three boys, ages 4, 7, and 10 years old, with gynecomastia. Blood levels of the patients' steroids, which are essential for the formation of hormones, were normal.

Bloch learned that the youngest boy's mother began applying a "compounded healing balm" containing lavender oil to his skin not long before his breasts started enlarging. The 10-year-old was using shampoo and styling gel that contained lavender oil and tea tree oil, and the 7-year-old was using lavender-scented soap and lotions.

In all three cases, the gynecomastia resolved within a few months after the use of these products was discontinued.

After discussing the cases with Bloch, co-author Dr. Kenneth Korach, from the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina, and colleagues conducted laboratory experiments using human breast cancer cells that expressed receptors of the female hormone estrogen, and breast cancer cells that expressed receptors of the male hormone androgen.

The results confirmed that the two oils weakly enhanced estrogen production, and also had anti-androgen effects, which together may have caused estrogen and androgen imbalances.

Further studies are needed to determine how often gynecomastia has been caused by lavender and tea tree oils, and if these oils have similar effects on men and in women and girls, the investigators caution.

Nevertheless, doctors should ask patients with gynecomastia about the skin products they are using, Korach comments in a press release. Prepubertal boys with gynecomastia may want to stop using products that contain these oils.

SOURCE: The New England Journal of Medicine, February 1, 2007.