An alternative steroid treatment to conventional HRT may increase the risk of womb cancer, say UK researchers.
There are pros and cons of taking HRT
A Cancer Research UK study of more than a million women found those on tibolone were more likely to get endometrial cancer than those on combined HRT.
However, combined HRT hiked up the risk of breast cancer, making it more risky overall, the authors told the Lancet.
Doctors questioned the validity of the work and stressed the cancer risks were very small.
They urged women not to be alarmed by the findings.
Hormone replacement therapy (HRT) is used to treat symptoms of the menopause, such as hot flushes, insomnia, headaches and irritability.
It can be given in varying strengths and combinations - oestrogen alone or combined oestrogen and progesterone.
Women who still have a uterus are often prescribed combined HRT, as oestrogen-only HRT is already known to increase the risk of womb or endometrial cancer.
Concerns about the safety of long-term HRT use have also grown after a series of studies linked it with a higher risk of breast cancer, heart attack and stroke.
On the flip side, there is some evidence to suggest that HRT protects against hip fractures and colon cancer.
Tibolone is a newer hormone treatment that is thought to have "sparing" effects on particular parts of the body, such as the breast, unlike regular oestrogen.
The Cancer Research UK team set out to look at the overall cancer risk posed by the different types of HRT available in the UK.
They asked postmenopausal women attending routine mammograms to fill out a questionnaire about HRT use.
They used this data to compare the rates of breast and endometrial cancers among women using no HRT, oestrogen-only HRT, combined HRT or tibolone.
As was expected, women taking oestrogen-only HRT were at greater risk of endometrial cancer. The same was true of women taking tibolone.
About six women out of every 1,000 taking tibolone, brand name Livial, developed cancer of the womb over a five-year period.
This compared with between two and three women per 1,000 for those on combination treatment - about the same cancer rate seen in women who have never taken HRT.
However, when rates for breast and endometrial cancer were taken together, the overall risk is highest in women taking combined HRT.
Study leader Professor Valerie Beral, director of Cancer Research UK's Epidemiology Unit, said: "These new results create a dilemma for women who haven't had a hysterectomy and want to use HRT.
"Since breast cancer is much more common than endometrial cancer, combined HRT poses the greatest overall cancer risk."
The Dutch pharmaceutical company, Organon, that makes Livial is now in talks with UK regulators about what advice to give patients and doctors.
A decision will later be made in agreement with the Medicines and Healthcare products Regulatory Agency (MHRA) on any changes to be introduced in light of the new evidence.
An MHRA spokeswoman said: "This adds important information to our growing knowledge of the effects of different types of HRT and underline the need for caution in long term use.
She said the data was unlikely to change the overall balance of risks and benefits for the short term use of HRT.
"Each decision to start or continue HRT should be made on an individual basis with a fully informed patient and should take into account any changes in her risk factors and personal preferences," sbe said.
Organon is currently running an international trial on the effects of Livial on the womb involving more than 3,000 women. The first results are expected towards the end of this year.
Mr Malcolm Whitehead, consultant at the menopause clinic at King's College Hospital in London, said the research was flawed because it did not take into account whether the women had switched or discontinued the type of HRT they were using at any time during the study.
He said that even if the findings were correct, the risks involved were very small.
Mr David Sturdee, consultant obstetrician and gynaecologist at Solihull Hospital, agreed, adding: "People should not panic."
He said many women benefited from HRT treatment.
Professor Gordon Duff, chairman of the Committee on the Safety of Medicines, which advises the government on medicine safety, said: "This latest data reinforces our previous advice that it is essential to consider all the risks and benefits when deciding HRT treatment for an individual woman."
He said women taking HRT need take no immediate action but should discuss their need for treatment with their doctor at their next review.