A study, published in The Lancet, showed women who took HRT to combat symptoms of the menopause even for a short period of time increased their risk of getting two common types of ovarian cancer by 40 per cent.
Researchers warned that taking HRT for less than five years, as is common medical practice, increases the risk of succumbing to the disease.
In Britain and the US alone about six million women are still using the treatment, which counters hot flushes, mood swings and other symptoms of the menopause.
Existing World Health Organisation, US and European HRT guidelines do not mention ovarian cancer, and UK guidelines state only that ovarian cancer might be increased with long-term use.
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An increase in risk was seen only for the two most common types of ovarian cancer, serous and endometrioid ovarian cancers, and not for the two less common types, mucinous and clear cell ovarian cancers.
Ovarian cancer is the fifth most common cause of cancer death among women in the UK, accounting for 6 per cent of all female deaths from cancer. Professor Dame Valerie Beral from the University of Oxford called for guidelines to be changed to inform women of the link between HRT and ovarian cancer.
She said: “The findings that ovarian cancer risk is greatest in current users of hormone therapy, falls after use ceases, and varies by tumour type strongly suggest a causal relationship – ie that among otherwise similar women, use of hormone therapy increases the probability of developing the two most common types of ovarian cancer, and hence ovarian cancer as a whole.
“The definite risk of ovarian cancer even with less than five years of HRT is directly relevant to today’s patterns of use – with most women now taking HRT for only a few years – and has implications for current efforts to revise UK and worldwide guidelines.”
The researchers analysed the results of 52 studies, involving a total of 21,488 women with ovarian cancer, almost all from North America, Europe and Australia
While the risk of ovarian cancer fell over time after treatment was stopped, women who had used HRT for at least five years still had a somewhat increased risk of ovarian cancer ten years later.
Professor Sir Richard Peto, of Oxford University, said: “For women who take HRT for five years from around age 50, there will be about one extra ovarian cancer for every 1,000 users and one extra ovarian cancer death for every 1,700 users.”
Target Ovarian Cancer’s head of research, Dr Simon Newman, welcomed the report but was keen to put the research findings into context.
He said: “The lifetime risk of developing ovarian cancer is one in 50, 2 per cent, and this study shows that five years use of HRT, starting at around 50 years of age, would result in around one additional case of ovarian cancer per 1,000 users of HRT.
“It is important to note that women treated for ovarian cancer who subsequently had HRT were not considered in this study.
“There currently isn’t any evidence that women treated for ovarian cancer who take HRT are at any additional risk of recurrence to the general population, but larger trials are required to confirm these studies.
“If a woman who has been treated for ovarian cancer has any concerns about the medication or treatment they are on, then it’s important to speak to their GP or consultant.”