WEDNESDAY, June 28 (HealthDay News) -- Personal-care products containing tea tree oils and lavender appear to cause abnormal breast development in pre-pubescent boys, a new study found.

The ingredients, which have an estrogen-like effect, are found in various shampoos, lotions, soaps and other products.

"There's definitely an association between exposure to lavender oil and tea tree oil and gynecomastia [abnormally large breasts in men]," said Derek Henley, lead author of the research and a postdoctoral fellow at the National Institute of Environmental Health Sciences, in Research Triangle Park, N.C.

Henley stopped short of declaring a cause-and-effect relationship, but did emphasize that the information could help physicians identify cases of gynecomastia that have no readily apparent cause.

"It may help steer physicians in a direction they wouldn't have thought of," Henley said. "The message is to raise awareness, so the public is aware of the association, and physicians are aware of it," he said.

Dr. Edward Reiter, chairman of pediatrics at Bay State Children's Hospital in Springfield, Mass., added, "The issue is a Sherlock Holmes kind of thing. This was a pretty good pick-up. It's old-fashioned science sleuthing. You have a problem, and you figure out what's going on."

Henley presented the findings at the just-completed annual meeting of the Endocrine Society, in Boston. Reiter was moderator of a press conference announcing the findings.

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.

And even though the condition is relatively uncommon, experts think this information will help clear other "mystery" cases.

"My guess is that this sort of thing occurs in a scattered way in little boys all the time," Reiter said. "It may well be that someone wouldn't even think of this. Now, doctors can ask if the patient is using any products," he said.

More information

The American Academy of Family Physicians has more on gynecomastia.