The UN's cancer agency has added hormone pills to the list of substances that can cause cancer.
In a review of existing research, the International Agency for Research on Cancer declared Friday that based on consistent evidence emerging from studies published over the last few years, it has reclassified hormonal menopause therapy from "possibly carcinogenic" to "carcinogenic."
The declaration from the World Health Organization, which is widely regarded as the international authority on which substances cause cancer, comes after several recent high-profile studies linking combination hormone replacement therapy, or HRT, to breast cancer.
The analysis, conducted by a panel of 21 scientists, concluded that estrogen and progestin menopause therapy also slightly increases the risk of endometrial cancer when progestin is taken fewer than 10 days a month.
Research has indicated that the chance of a woman developing breast cancer during her life is one in seven.
A landmark study used in the UN agency's analysis suggested that long-term use of the hormones raises the lifetime risk of breast cancer to about one in six.
The UN cancer agency also concluded that a common type of contraceptive pill, taken by about 10 percent of women of reproductive age, increases the risk of more types of cancer than previously thought.
The agency had previously determined that the pill combining estrogen and progestin can cause liver cancer. Now, further research has demonstrated that it also slightly increases the risk of breast and cervical cancer, the agency said.
"It's a complicated picture," said Vincent Cogliano, head of the agency's department that evaluates the cancer risk of chemicals.
The increased cancer risk from the birth control pill was small and transient, the analysis found.
While experts did not dispute the agency's conclusions on hormone replacement therapy, some were less convinced that current evidence proves the contraceptive pill is carcinogenic and urged caution in interpreting the conclusions.
"I think it's flat-out wrong," said Dr. Steven Goldstein, professor of obstetrics and gynecology at New York University Medical Center. "Most of the studies were using pills two-and-a-half to four times the dosage that I'm using today in most women."
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