THURSDAY, Aug. 7 2003 (HealthDayNews) --

Not only does hormone replacement therapy (HRT) increase the risk of breast cancer for women taking the supplements, but those tumours appear to be especially deadly.

That's the conclusion of a major, five-year study of more than 1 million British women, and it represents the third strike this week against HRT.

The researchers found that those taking combination HRT had a higher risk of developing breast cancer and a greater risk of dying from the disease than did women not using the therapy.

Previous studies, including the Women's Health Initiative in the United States, have identified the link between breast tumours and HRT. But the British research is the first to suggest that the tumours caused by hormone supplements are more aggressive.

The new work, reported in the Aug. 9 issue of The Lancet, found that the breast cancer risks are magnified the longer women take hormone treatments -- but it also found that they fade with time after the therapy is stopped. Over the last decade, the researchers estimate that use of HRT in the U.K. has led to 20,000 extra cases of breast cancer among women ages 50 to 64.

In an editorial accompanying the journal article, three doctors from the Netherlands and Canada advised most women taking HRT to stop doing so immediately. "Now there's some urgency to get these people off" hormone therapy, says Dr. Walter Rosser, head of family medicine at Queens University, in Kingston, Ontario, and a co-author of the commentary.

Between 20 percent and 50 percent of women in the Western world who are between the ages of 45 and 70 have taken or are now taking HRT, according to Rosser.

So if the new study is accurate, he says, there will be an extra case of breast cancer in every 166 women who take the hormones for five years, and an extra case in every 52 women who do so for a decade. "That's something you have to take pretty seriously," he adds.

Rosser acknowledges that women with particularly severe symptoms of menopause may choose to take HRT despite the breast cancer risks it carries. "I would give it to them," he adds, "but probably for only six months at a time" to see how they feel when they stop the treatment.

However, Dr. Wulf Utian, executive director of the North American Menopause Society, says calls for women to stop taking hormone therapy are "radical" and equivalent to "shouting 'Fire!' in a crowded movie theatre."

The menopause group will be issuing guidelines next month on how doctors should prescribe hormone replacement therapy, says Utian, who adds that he's not an "oestrogen evangelist." Until then, women don't need to take any drastic measures, he says.

Doctors in the United States prescribe hormone replacement therapy to women who have undergone menopause for two reasons. One is to control symptoms like hot flashes and vaginal dryness associated with the loss of oestrogen that marks menopause. Another is to prevent bone fractures from osteoporosis, another consequence of declining levels of the female sex hormone.

Until recently, many women had also been taking HRT to prevent heart and vessel disease, on the strength of studies suggesting such a benefit.

But just this week new evidence in two studies shows that the therapy doesn't help, and can even harm, the heart.

One showed that HRT doesn't slow the advance of atherosclerosis -- a build-up of fatty deposits in the arteries -- in women who already have the condition. The second, part of the Women's Health Initiative, found that HRT poses the greatest risk of heart attack during the first year of use. Both studies appear in the Aug. 7 issue of the New England Journal of Medicine

The latest study, called the Million Women Study, was done between 1996 and 2001 by Cancer Research UK, an epidemiology unit based in Oxford. It followed almost 1.1 million British women who were 50 to 64 years old, of whom roughly half had taken or were taking hormone supplements. These included a mix of oestrogen and progestin -- to avoid uterine cancers triggered by oestrogen -- and oestrogen alone. Some were also taking a drug called tibolone, which is a precursor to oestrogen, progestin and other sex hormones. Tibolone is not sold in the United States.

Women on oestrogen-progestin therapy had twice the risk of developing breast cancer during the five-year study as those who'd never taken the hormones. Those on oestrogen alone had a 30 percent increased risk of tumours, while those on tibolone had a 45 percent increased risk. Taken together, current users of HRT were about two-thirds more likely than non-users -- including those who'd once taken the therapy but dropped it -- to develop breast cancer, the researchers say.

Women on HRT were also 22 percent more likely than those never on the drugs to die of breast cancer. In all, 637 women died of the disease during the study.

Breast cancers were more common among HRT users regardless of the kind of hormones used or how they were taken, orally or through a skin patch.

The risk also increased with duration of treatment, so that staying on oestrogen alone for 10 years would lead to five extra cases of breast cancer per 1,000 women. That figure climbs to 19 extra cases per 1,000 for the combination of oestrogen and progestin, the researchers say. Both numbers are consistent with previous findings of increased breast cancer risk in the Women's Health Initiative.

In an unrelated study in the same issue of The Lancet, European researchers found that oral oestrogen, but not the skin patch form, increased a woman's risk of leg clots. The risk was about 3.5 times greater for women on oestrogen than for those not taking the hormone.