Boys have been warned against using oils or hair gels that contain lavender or tea tree oil after three reported cases of them growing breasts.

Researchers in the US believe that the oils may have hormone-like properties that lead to gynecomastia — the growth of breasts. When the boys stopped using the oils, the breasts disappeared. Writing in the journal New England Journal of Medicine, the researchers say that the repeated use of such oils may disrupt normal hormonal function.

One of the boys was four years old, and his mother said that she had applied a lavender-based “healing balm” to his skin. A second boy, aged 10, had regularly used a styling gel containing lavender and tea tree oil, applying it to his hair and scalp. The third boy, who was 7, had used lavender soap and skin lotions. In all cases the problem was resolved within a few months of giving up the products.

The researchers, from the US National Institutes of Health and the paediatric department of the University of Colorado School of Medicine, say that the oils “may possess endo-crine-disrupting activity that causes an imbalance in oestrogen and androgen pathway signalling”.

They investigated this by growing breast cancer cells in test tubes and adding diluted extracts of lavender and tea tree oil. These experiments showed that the oils had activity that mimicked the female sex hormone oestrogen.

To have the effects found in the three boys, the oils would need to have penetrated the skin, and the team did not investigate whether this was possible.

They point out that the development of breasts is not uncommon during puberty in boys, but is extremely uncommon before puberty. There is no known physiological cause.

Usually, some exposure to an environmental chemical is identified as the cause. This can also happen, very occasionally, in older men, the researchers say.

They said: “This report raises an issue of concern, since lavender oil 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.”

Because of the small number of cases that were studied, further work was needed to confirm the link, they said.

Products that contain the oils are usually sold as natural aids to relaxation or skin care. Tea tree oil comes from the leaves of a plant called Melaleu-ca alternifolia, native to the coast of New South Wales.

It is claimed to have beneficial cosmetic and medical properties, including combating bacterial and fungal infections. Lavender oil is used in aromath-erapy and massage oils.

A spokeswoman from Boots, the pharmacist, said that the store sold pure lavender and tea tree essential oils, “but as with any essential oil we don’t recommend them for use on children or pregnant women”.

The store was not withdrawing products or solutions containing the oils, or issuing safety warnings, because the chances of receiving significant enough doses to cause hormone imbalance through the skin was unlikely, she said.

“This is a preliminary study. As long as customers follow advice about how to use cosmetics and medicines correctly, we regard our products as safe.”

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