The United Nations cancer agency has concluded that hormone-replacement therapy, taken by millions of women around the world, causes cancer, but it said that does not mean women should automatically stop the treatment.

In a monograph published Friday, the International Agency for Research on Cancer declared that, based on consistent evidence emerging from studies over the past few years, it has reclassified hormonal menopause therapy from "possibly carcinogenic to humans" to "carcinogenic to humans."

Studies have convincingly shown that the treatment slightly increases the risk of breast and endometrial cancers, the agency determined.

It also concluded that the combined contraceptive pill, taken by about 10 percent of women of reproductive age, causes more types of cancer than previously thought.

The pill had been identified as causing liver cancer. Further research has demonstrated that it also slightly increases the risk of breast and cervical cancer, the agency said. The investigation also confirmed, however, that the pill decreases the risk of endometrial and ovarian cancers.

"It's a complicated picture," said Vincent Cogliano, head of the agency's monographs department, which is widely regarded as the international authority on which substances cause cancer. "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."