Women who took hormone replacement pills had more advanced breast cancers and were more likely to die from them than women who took a dummy pill, raising new concerns about the commonly prescribed drugs, researchers said.
The study, published in the Journal of the American Medical Association, is the first to report more breast cancer deaths among women taking hormone replacement therapy.
And it contradicts prior studies that suggest women taking the drugs had less aggressive, easier-to-treat breast cancers.
"As opposed to the prevailing thought of two years ago, that cancers associated with oestrogen plus progesterone would be favourable and not much of a problem, we are actually showing they are associated with an increased risk of death from breast cancer," Dr Rowan Chlebowski of the Los Angeles Biomedical Research Institute, who led the study, said.
The findings include 11 years of follow-up from the Women's Health Initiative study, which in 2002 found women who took oestrogen plus progesterone for five years had higher rates of ovarian cancer, breast cancer, strokes and other health problems.
Twice as many cancer deaths
Chlebowski's team analysed data on the more than 12,000 women in the study. They found twice as many taking HRT died from breast cancer - 2.6 per 10,000 per year versus 1.3 per 10,000 women per year - compared to women who took a placebo.
Nearly 24% of the breast cancer patients who took HRT had tumours that had spread to the lymph notes, compared with 16% of women taking placebos.
"All the scary cancers with unfavorable prognoses were also increased," Chlebowski said, citing increases in aggressive forms of breast cancer, and not just oestrogen-fed cancers that are easier to treat.
Deaths from breast cancer
"And then for the first time we show deaths from breast cancer are significantly increased as well," he said.
Pfizer said in a statement the company stands behind Prempro's current labeling, which advises doctors to prescribe the drug at the lowest effective dose for the shortest possible period of time.
But even that may be risky, suggests Dr Peter Bach of Memorial Sloan-Kettering Cancer Centre in New York, who wrote a commentary in the same journal.
Doctors can only be guessing that taking the pills at a lower dose and for a shorter time would be less harmful, Bach said.
Doctors should be aware
Doctors "should be aware that this approach has not been proven in rigorous clinical trials," Bach wrote.
"As a science-based company, we take this analysis seriously," Pfizer said in a statement. "It is important to view the data in the full context of both the symptoms of menopause as well as the extensive body of information - developed over more than 60 years - on the known benefits and risks of hormone therapy."
Doctors note that the average age of the women in the Women's Health Initiative study was 63, several years past menopause, and say the findings may not apply to women taking other forms of HRT, or to those starting HRT immediately at the time of menopause. (Reuters Health/ October 2010)
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