Parabens used in cosmetics and personal care products may stimulate the growth of breast cancer cells, according to a new study.

Parabens are a class of preservatives widely used in consumer products. They are considered estrogenic because they activate the same estrogen receptor as the natural hormone estradiol.

Studies have linked exposure to estradiol and related estrogens with an increased risk of breast cancer, as well as reproductive problems. “Although parabens are known to mimic the growth effects of estrogens on breast cancer cells, some consider their effect too weak to cause harm,” said lead investigator Dale Leitman, from University California, Berkeley in US.

“But this might not be true when parabens are combined with other agents that regulate cell growth,” said Leitman.

Existing chemical safety tests, which measure the effects of chemicals on human cells, look only at parabens in isolation and fail to take into account that parabens could interact with other types of signalling molecules in the cells to increase breast cancer risk.

In the study, researchers looked at breast cancer cells expressing two types of receptors – estrogen receptors and human epidermal growth factor receptor 2 (HER2).

Approximately 25 per cent of breast cancers produce an abundance of HER2. HER2-positive tumours tend to grow and spread more aggressively than other types of breast cancer.

The researchers activated the HER2 receptors in breast cancer cells with a growth factor called heregulin that is naturally made in breast cells, while exposing the cells to parabens.

Not only did the parabens trigger the estrogen receptors by turning on genes that caused the cells to proliferate, then effect was significant.

The parabens in the HER2-activated cells were able to stimulate breast cancer cell growth at concentrations 100 times lower than in cells that were deprived of heregulin.

The study demonstrates that parabens may be more potent at lower doses than previously thought, which may spur scientists and regulators to rethink the potential impacts of parabens on the development of breast cancer, particularly on HER2 and estrogen receptor positive breast cells.