Women treated with the most commonly used form of hormone replacement therapy are three times more likely to develop breast cancer than those who do not use it, according to a new British study.

The findings, published in the British Journal of Cancer, suggest the risk of HRT has been previously understated by as much as 60 percent, The Guardian reports,

“What we found is that the risks with combined HRT are larger than most of the literature would suggest,” said Anthony Swerdlow, professor of epidemiology at the Institute of Cancer Research (ICR) in London, which led the study.

For the study, researchers tracked 39,000 women for six years. During that time 775 developed breast cancer and women using a combination of estrogen and progestogen were 2.7 times more likely to develop cancer than who had never used HRT.

No increase in risk was observed in women using estrogen-only HRT.

About one in 10 women in their 50s use HRT to deal with menopausal symptoms including hot flushes, night sweats, insomnia, mood swings, and tiredness.

Health experts have long debated the risk of breast cancer from HRT, which could be explained by an increased exposure to hormones that fuel the growth of some tumors.