Tea tree and lavender oil found in soaps, shampoos and hair gels may be causing men to grow breasts, research has suggested.
Petri-dish analysis revealed that some of the chemicals widely used in the essential oils promoted the female hormone oestrogen, while holding back testosterone.
Previously, scientists have suspected a link between use of these products and cases of gynaecomastia, commonly referred to as "man boobs".
However, the new laboratory tests are the first time that researchers have witnessed their endocrine-disrupting qualities at work in this way.
The team in charge of the experiments has called for better regulation of substances containing the essential oils, and for further studies into the long-term effects of exposure.
Gynaecomastia is most common in teenage boys and older men, symptoms vary from a small amount of extra tissue around the nipples to more prominent breasts. It can cause breast tissue to be tender or painful.
A growing number of reported cases of gynaecomastia have coincided with topical exposure to the oils, with symptoms subsiding after the subjects stopped using the products.
Jeffrey Ramsey, who led the research at the National Institute of Environmental Health Sciences in North Carolina, said: "Essential oils possess a diverse amount of chemicals, and should be used with caution because some of these chemicals are potential endocrine disrupters."
His team singled out eight chemicals from the hundreds found in the oils - half appeared in both tea tree and lavender and half in only one.
They then applied the chemicals to human cancer cells, finding all eight promoted oestrogen and inhibited testosterone.
Professor Ieuan Hughes, emeritus professor of paediatrics at the University of Cambridge, said: "It is possible there are individuals who may be more sensitive to the effects of the chemicals."
The Aromatherapy Trade Council advises that essential oils should not be applied undiluted directly to the skin.
The study will be presented at the Endocrine Society's annual meeting in Chicago this week.