CHICAGO ( Reuters ) -

Women who are on hormone-replacement therapy have a much higher risk of developing asthma late in life, adding to the long list of health woes caused by the once-popular treatment for menopausal symptoms.

The research, published todayin the Archives of Internal Medicine, found that women who take a combination of estrogen and progestin, or even estrogen alone, more than double their risk of developing asthma.

The study showed that HRT does not influence women's risk of developing chronic obstructive pulmonary disease, a serious lung disorder most commonly attributed to smoking.

"The data suggest that for women who have severe asthma that develops late in life, they may want to consider a trial of stopping hormone-replacement therapy to see if this alleviates their condition," said lead author Graham Barr, a researcher at Brigham & Women's Hospital in Boston.

He said the onset of asthma late in life is a rare occurrence, but it can be a severe condition and, as such, "this finding presents yet another factor women may need to consider when debating whether or not estrogen still has a role in the short-term treatment of menopausal symptoms."

Combination HRT has, in recent years, been linked to a long list of problems, including increased risk of heart disease, stroke and breast cancer. But it is still seen as effective and relatively safe for short-term relief of menopausal symptoms including hot flashes, insomnia and vaginal dryness.

The new research is unusual in that it also looked at the risk of estrogen alone. Dr. Barr said the research is also important because it strongly suggests that sex hormones play a role in asthma, a condition that affects more than 5 per cent of the population.

Estrogen, a female sex hormone, is known to influence inflammation, which plays a key role in asthma. The researchers also speculate that estrogen may influence the production of nitric oxide, which also plays a role in the development of asthma.

The study notes that when post-menopausal women develop asthma, the condition usually clears up if they stop HRT, but that can take up to four years to occur.

The research came out of the Nurses' Health Study, an on-going project that has followed more than 120,000 nurses since 1976, tracking their histories, diets and lifestyles.

About 40 per cent of women using HRT stopped or reduced their use of the drugs since a study calling the therapy into question was published in July of 2002; but about one in four of those resumed because their symptoms were too troublesome without HRT.

In Canada, there are about five million women over age 50; about 1.2 million of them were taking HRT before the study was published. Menopause occurs at the end of a woman's last menstrual period, when ovulation ceases. A woman's ovaries make less of the female hormones -- estrogen and progesterone -- and that can lead to symptoms including hot flashes, mood swings, vaginal dryness, insomnia and mild memory loss.