LONDON (Reuters) - The synthetic hormone Livial, designed as an alternative to hormone replacement therapy, makes it significantly more likely a woman's breast cancer will come back, Dutch researchers said on Tuesday.

Breast cancer was 40 percent more likely to come back in woman taking Schering Plough Corp's (SGP.N) Livial, a finding so pronounced the researchers said they stopped the trial six months early.

About 70 percent of the recurrent tumours killed the patients.

The hormone, known generically as tibolone and marketed mostly in Europe, is licensed in 90 countries to ease the hot flushes, night sweats and bone loss associated with menopause, while 55 countries have approved it for osteoporosis, according to the study published in the journal Lancet Oncology.

It is a so-called selective tissue estrogenic activity regulator, meant to act like estrogen to strengthen bones and prevent hot flashes while avoiding estrogen's known cancer-promoting effects in the breast.

Many breast cancer patients currently use the drug for menopause symptoms, but the findings show doctors should not prescribe the medicine to cancer survivors, Peter Kenemans of the VU University Medical Centre in Amsterdam and colleagues said.

The tibolone hormone can mimic the effects of both estrogen and progestogen, which means women only need to take the one hormone instead of two.

The most likely explanation, the researchers said, is that the hormone interferes with the protective effect of different cancer drugs and may stimulate dormant tumours.

The findings of this trial will provide a better basis for general practitioners, gynaecologists, oncologists, and other doctors when counselling patients with breast cancer who are severely symptomatic with hot flushes and night sweats that interfere with sleep and impair quality of life, they wrote.

Hormone replacement therapy was popular until 2002 when a large study called the Women's Health Initiative study suggested it could raise the risk not only of breast and ovarian cancer, but also strokes and other serious conditions.

The findings spurred millions of women to abandon HRT and hit the shares of makers of hormone therapies such as Wyeth (WYE.N).

Kenemans and his team assessed whether a dose of 2.5 milligrams per day of Livial increased the risk of breast cancer recurrence in more than 3,000 women who had breast tumours removed. Half received the drug and the others got a placebo.

The findings suggest there is no group of breast cancer patients for whom Livial is safe and doctors should stop giving the drug to these women, the researchers said.

This is an important question for both doctors and patients, because many patients with breast cancer with bothersome complaints that do not respond sufficiently to non-hormonal treatment seek aid in the form of off-label use of tibolone, the researchers wrote.

A spokeswoman for Schering-Plough said the study's data was well known, and the company had never marketed the product to women with breast cancer anyway. (Reporting by Michael Kahn; editing by Maggie Fox and Will Waterman)