The study found few differences in risk between the formulations; Women can not protect themselves by using implants or intrauterine devices that release a hormone directly into the uterus.
The research also suggests that the hormone progestin, widely used in current contraceptive methods, may be increasing the risk of breast cancer.
"This is an important study because we had no idea how modern pills compared with outdated pills in terms of breast cancer risk, and we did not know anything about the IUD," said Dr. Marisa Weiss , an oncologist who founded the website breastcancer.org and did not participate in the study. "The gynecologists simply assumed that a lower dose of hormone meant a lower risk of cancer, but the same high risk exists."
"It's small but it can be measured, and if you add up all the millions of women who take the pill, it's a major public health problem," Dr. Weiss added.
Officials at the American College of Obstetricians and Gynecologists said Wednesday they will carefully evaluate the new findings, but emphasized that hormonal contraceptives are for many women "one of the safest, most effective and accessible options available."
Experts noted that oral contraceptives also have some benefits, and are associated with reductions in ovarian, endometrial, and colorectal cancers later in life.
Dr. Chris Zahn, vice president of practice at A.C.O.G., recognized a link between the risk of breast cancer and the use of hormones, but urged concerned women to consult a trusted medical provider before making changes. "It is important that women feel safe and comfortable with their contraceptive choice," she said.
Because the risk increases with age, Dr. Weiss suggested that older women want to consider switching to a hormone-free contraceptive method, such as a diaphragm, an IUD that does not release hormones or condoms. "It's not like you have no other choice," he said. "Why not look for another option?"
In a commentary accompanying the new study, which was published in the New England Journal of Medicine, David J. Hunter, professor of epidemiology and medicine at the University of Oxford, said that the new study did not find any contraceptive modern was free of risks.
"There was a hope that contemporary preparations would be associated with a lower risk," he said in an interview. "This is the first study with substantial data to prove that this is not the case."
Nearly 10 million American women use oral contraceptives, including about 1.5 million who depend on them for reasons other than birth control. The number of women in the United States with intrauterine devices, many of whom release hormones, has grown in recent years, as has the number of women who use other types of hormonal contraceptive implants.
Although it was known that older oral contraceptives increased the risk of breast cancer, many doctors and patients had assumed that the new generation of pills on the market today were safer. However, the new study found increased risks that were similar in magnitude to the higher risks reported in previous studies based on contraceptive pills used in the 1980s and earlier, said Dr. Hunter.
"We really expected that we would find a smaller increase in risk because today we have lower doses of estrogen in hormonal contraceptives, so it was surprising to find this association," said Lina S. Mørch, principal investigator at the University of Copenhagen and main author of the article.
The study also found that the risk increased the longer women used hormonal contraceptives, suggesting that the relationship is causal, Dr. Mørch said. "It's a very clear picture for us, very convincing."
The study, which was used throughout Denmark as a sample, followed about 1.8 million women of childbearing age for more than a decade, obtaining data from the national prescription and cancer registers. During that period, 11,517 cases of breast cancer were identified.
The researchers concluded that hormone users experienced a 20 percent increase in the relative risk of breast cancer compared to non-users, although the risk increased with age and varied according to the formulation. Still, the additional risk would result in a few additional cases of breast cancer, the researchers said.
The increase in cases of breast cancer associated with hormones was also small because to begin with, young women are at low risk. But the odds increased among women who used hormonal contraceptives for more than 10 years, the study found. Among those who used hormones for five years, the increased risk of breast cancer persisted even after discontinuing its use, said Dr. Mørch.
What really surprised the researchers was that the increased risk was not limited to women who used oral contraceptive pills. It was also seen in women who used implanted intrauterine devices, or IUDs, that contain the hormone progestin. (Not all release hormones from the U.D.)
Women who used an intrauterine device that releases progestin alone also faced a 21 percent increase in risk, compared to those who did not use it, the study found. The findings indicate that the hormone progestin increases the risk of breast cancer; Some of the contraceptive pills and many of the intravenous drugs include progestin alone, said Dr. Mørch.
A 20 percent increase in relative risk may be small in absolute terms, but the calculation changes with age. For a 20-year-old woman, for example, the probability of developing breast cancer in the next 10 years is .06 percent, or 1 in 1,732, according to breastcancer.org.
Even if the relative risk increases by 20 percent, it is still less than one-tenth of 1 percent. But when a woman reaches 40, her chance of developing breast cancer in the next 10 years is 1.45%, or 1 in 69. An increase of 20% increases her risk to 1.74%, or 1 at 57.
Women who remained on hormones for 10 or more years experienced a 38 percent increase in their relative risk of developing breast cancer, compared to non-users. In contrast, there was no increased risk of breast cancer in women who used hormones for less than a year.
In Denmark, older women who have completed their family are more likely to use the IUD, including those that contain hormones, and are already more likely to develop breast cancer because of their age, Dr. Mørch said.
"Nothing is risk free, and hormonal contraceptives are no exception to that rule," said Dr. Øjvind Lidegaard, lead author of the article. But he suggested that doctors take time to analyze the pros and cons of different types of contraception with their patients, and be candid about the potential risks, suggesting that women reevaluate the use of hormones as they get older.
The study was funded by a grant from the Novo Nordisk Foundation, which describes itself as "an independent Danish foundation with corporate interests", supporting medical research in public institutions and by companies within the Novo Group. (Novo Nordisk is a global healthcare company that focuses on diabetes care and also manufactures Vagifem, topical estrogen inserts designed for menopausal women.)
A postscript with the article noted that two of the study's authors , including Dr. Mørch, have been employed by Novo Nordisk since the manuscript was accepted for publication.
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