Overall, the study found that women who used birth control had a 20 percent increase in their relative risk for developing breast cancer.
The study, which followed 1.8 million Danish women for more than a decade, upends widely held assumptions about modern contraceptives for younger generations of women.
The bottom line is that before starting or continuing to take hormonal contraceptives - or any medications - it's important to speak with your doctor about any potential risks and benefits, and make an informed decision from there. Still, the data show that the search for birth control drugs that don't increase breast cancer risk must continue, he said.
A new study is showing the link between the use of birth control to increased risk of breast cancer.
The study found few differences in risk between the formulations; women can not protect themselves by turning to implants or intrauterine devices that release a hormone directly into the uterus.
In fact, birth control increases breast cancer risk about as much as drinking alcohol does, said Dr. Mary Beth Terry, an epidemiologist at the Columbia University Mailman School of Public Health.
"Gynecologists just assumed that a lower dose of hormone meant a lower risk of cancer. If you have a reason to be taking them, it's perfectly reasonable to do so".
"There's very good evidence", Hunter says, "that oral contraceptives reduce the risk of ovarian cancer". But if they had taken hormonal contraception for more than five years, the higher risk of breast cancer persisted for at least five years after their discontinuation of hormonal birth control, the study found. "It's not like you don't have a choice", she said.
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"There was a hope that the contemporary preparations would be associated with lower risk", he said in an interview. But these earlier studies looked mainly at older types of birth control pills, which had a higher dose of estrogen than today's pills.
It's always been known that hormonal contraception, like any medicine, carries some risks. However, using hormonal contraception for 10 years was linked with a 40 percent increase in the risk of breast cancer, compared with those who had never used hormonal contraception. Risk increased with longer use, from a 9 percent increase in risk with less than a year of contraceptive use to a 38 percent increase after more than 10 years of use.
Researchers analyzed health records of 1.8 million women, ages 15 to 49, in Denmark where a national health care system allows linking up large databases of prescription histories, cancer diagnoses and other information. During that time period, 11,517 cases of breast cancer were identified.
PATTI NEIGHMOND, BYLINE: Scientists have long known contraceptives that contain estrogen can increase the risk of breast cancer.
Still, the researchers noted that any unaccounted factors would need to have a large effect on the risk of breast cancer and be very common in the population to explain the results. A 20 percent increase translates into only one extra breast cancer case for almost every 8,000 women. But the odds rose among women who used hormonal contraception for more than 10 years, the study found. "So, many calculations suggest that the use of oral contraceptives actually prevents more cancers than it causes".
"In the 1980s and 1990s, there was some optimism regarding the development of a formulation that would reduce a woman's risk of breast cancer", he said in the commentary, "but research into this possibility appears to have stalled". By contrast, there was no increased risk for breast cancer seen in women who used hormones for less than one year.
"Most of the cases that occurred in this analysis occurred among women who were using oral contraceptives in their 40s", Hunter adds.
"Nothing is risk-free, and hormonal contraceptives are not an exception to that rule", said Dr. Øjvind Lidegaard, the paper's senior author.