The risk of developing breast cancer as a result of Hormone Replacement Therapy (HRT) is twice what was previously thought.
New findings from a study carried out by the University of Oxford and published in The Lancet have prompted researchers to warn that the official guidance on prescribing HRT should be reconsidered.
There are over 12 million users of HRT in Western countries with data from 100,000 of those users taking part in the study.
The study found that the risk of cancer persists for 10 years after stopping the use of HRT with the risk of breast cancer doubling after 10 years of use.
Head of Research at the Irish Cancer Society, Robert O'Connor, is urging women not to panic.
He says there's still evidence to suggest HRT is a safe and effective treatment for some women going through the menopause.
I think the advice for women is if they have any concerns, they haven't already gone through these risks, to have a chat with their GP and understand the risk, which we have known about for some time.
"Understand that risk against the background of their overall health risks and all the other different factors that are involved."