The International Agency for Research on Cancer (IARC), which collates existing oncology studies, declared that consistent evidence emerging from studies in the past few years has lead it to reclassify hormonal menopause therapy from "possibly carcinogenic to humans" to "carcinogenic to humans."

The agency said studies have convincingly shown that doses of estrogen and progestogen in combination slightly increase the risk of breast cancer. And estrogen-only treatment, while posing less of a risk for breast cancer, can cause endometrial cancers.

Decisively Labeled 'Carcinogenic'

A major US study into HRT in 2002 established a link to breast cancer, while links to endometrial cancers have long been known. But it is the first time the respected international agency has decisively labeled the treatment carcinogenic, effectively ending any remaining doubt.

Dr. Peter Boyle, director of IARC, said: "[This report] address exposures that are experienced daily by many millions of women worldwide. It is of enormous public health importance that we identify and understand the full range of effects of these products."

HRT is used to treat women suffering from symptoms caused by perimenopause -- the transition before menopause when estrogen and progesterone levels drop. Symptoms include insomnia, mood swings, hot flashes, reduced mental clarity, vaginal dryness and heavy menstrual periods.

In 2000, approximately 20 million women in developed countries used HRT, although that number has dropped in recent years as the treatment has been linked to cancer. Many doctors continue to prescribe a low-dose therapy for severe cases, but symptom-specific treatments such as lubricants, blood pressure medication and anti-depressants are becoming increasingly popular.

Pill Causes More Types of Cancer

The report also concluded that the combined contraceptive pill causes more types of cancer than previously thought. Previously, the pill had been identified as causing liver cancer. Now, further research has demonstrated that it also slightly increases the risk of breast and cervical cancer, the agency said.

However, the investigation also confirmed that the pill decreases the risk of endometrial and ovarian cancers.

Vincent Cogliano, head of the agency's monographs department, which is widely regarded as the international authority on which substances cause cancer, said: "It's a complicated picture. It doesn't mean women should stop taking the treatment. There are still other reasons to take it. Each woman has to discuss it with her doctor and weigh the risks and benefits."

Worldwide, more than 100 million women -- about 10 percent of all women of reproductive age -- currently use combined hormonal contraceptives. Commonly known as "the pill," but also administered through patches and injections, the treatment prevents pregnancy by stopping ovulation and thickening the mucous of the cervix so sperm cannot get through.

Combined hormonal contraceptives can cause weight gain, breast tenderness, depression and a decreased interest in sex. But it has also been shown to reduce the risk of noncancerous breast cysts, infections of the fallopian tubes, iron deficiency anemia, and ectopic, or tubal, pregnancies.