The U.N.’s cancer research agency added hormone pills Friday to the list of substances that can cause cancer.
The International Agency for Research on Cancer said that based on consistent evidence from studies in recent years, it was reclassifying hormonal menopause therapy from “possibly carcinogenic” to “carcinogenic.”
The declaration from the World Health Organization’s cancer agency, which is widely regarded as the international authority on cancer-causing agents, 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 therapy for menopause 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 lifetime is about 1 in 7. A landmark study used in the U.N. agency’s analysis suggested that long-term use of hormones raises the lifetime risk to about 1 in 6.
The cancer research agency also concluded that a common type of birth-control pill, taken by about 10 percent of women of reproductive age, increases the risk of more types of cancer than previously thought.
Some doctors cautioned that the conclusion was based on studies of higher-dosage pills and said they wouldn’t change their advice to young women.
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.
However, the investigation also confirmed that the pill protects against endometrial and ovarian cancers.
“It’s a complicated picture,” said Vincent Cogliano, head of the agency’s department that evaluates the cancer risk of chemicals.
“There are still other reasons to take it. Each woman has to discuss it with her doctor and weigh the risks and benefits” for either the contraceptive pill or hormone replacement therapy.”