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Essential oils from plants are touted as having many beneficial properties. But just because they’re natural doesn’t mean they’re harmless – two commonly used plant oils seem to mimic female hormones in the body, occasionally causing boys to grow breasts.

Chemicals from lavender are often used in toiletries for their scent, and are said to aid relaxation. Tea tree oil is a mild antiseptic and a common ingredient of children’s hair products as it is claimed to prevent head lice, although there is .

But both of these may have side effects. In 2007, doctors reported three cases of prepubescent boys , despite having the expected hormone levels for their sex and age.

Breast growth in all three boys began when they started using products such as soap or shampoo that contained lavender or tea tree oils. A similar effect has been reported with a , however .

Oestrogen effect

To investigate this, of the National Institute of Environmental Health Sciences in Durham, North Carolina and his team have been investigating eight chemicals that must be present in a product for it to be marketed as containing either of these plant oils.

When applied to human breast cancer cells in the lab, depending on the test used, at least seven of these chemicals changed the activity of genes that normally respond to oestrogen, the team will tell a meeting of the   on Monday. Many of these substances are also listed as components of other plant oils, says Korach.

He says girls may also be susceptible to the hormone-disrupting effects, but any breast growth in girls could be mistaken for early puberty.