Gynecomastia is associated with an imbalance of estrogens (so-called female hormones) and androgens (so-called male hormones). Young boys don't produce much testosterone, so there is nothing in the body to counteract extra estrogen, such as might come from mistakenly taking a mother's birth control pill or, more concerning, an estrogen-secreting tumor.
The condition is unusual. "I may see a patient every year or every other year with this, a pre-pubertal boy who has some breast development," Reiter said. "Usually, it's a tiny amount, and you never find out what it is, then it goes away," he explained.
But when Dr. Clifford Bloch, co-author of the study and a pediatric endocrinologist in Denver, saw five boys with the problem, he decided to investigate. All of the boys, he soon realized, had used over-the-counter health-care products containing lavender and/or tea tree oil on their hair or skin.
When tested on human breast cancer cells, the oils appeared to mimic the activity of estrogen and also suppressed male hormone responsiveness. After discontinuing use of the lavender and tea tree oils, all of the boys saw the problem resolved.