(CNN) - Birth control can increase a woman's risk of breast cancer by up to 38%, depending on how long she has taken it, a new study finds.
The risk was associated with all forms of hormonal contraception -- such as the pill, injections or IUDs -- when compared with women who have never used them.
Researchers from the University of Copenhagen analyzed data from 1.8 million women under the age of 50 in Denmark. They followed the women for nearly 11 years, on average.
The level of breast cancer risk increased the longer a woman had been taking hormonal contraceptives, with the average risk increase being 20% among all current and recent users of these forms of contraceptives.
The researchers saw a 9% increased breast cancer risk among women taking hormonal contraceptives for under a year, rising to 38% if more than 10 years.
Among women who had been using hormonal contraceptives for more than five years, a slight risk persisted for at least five years after they stopped, according to the study published Wednesday in the New England Journal of Medicine.
The results suggested a "rapid disappearance of excess risk of breast cancer after discontinuation of use among women who have used hormonal contraceptives for short periods," the authors write in the paper. The authors noted that other studies have found no evidence of a persistent risk.
In an accompanying editorial, David Hunter, professor of epidemiology and medicine at the Nuffield Department of Population Health in the UK, said that the link between oral contraceptives and breast cancer is already well-established. However, this new study is important because it looked at newer preparations of contraceptives, he told CNN.
"These results do not suggest that any particular preparation is free of risk," he wrote in the editorial.
However, Hunter also stresses that "breast cancer remains a relatively rare disease in younger women." In women under 35 included in the study, taking hormonal contraceptives for less than one year resulted in 1 extra case per 50,000 women, he said.
"The number of cases increases with age because the risk of breast cancer increased with age," said Hunter.
Hunter also adds that the persistent risk found in the study "should be regarded as preliminary," with the increase unlikely to be significant when other factors, such as different durations of use and time since last use, are accounted for.
"The risk does decline over time since ceasing their use," said Hunter, highlighting that once women reach the age when breast cancer rates peak -- ages 50 to 70 -- they are not very influenced by whether they took the hormonal contraceptives.
Low breast cancer rates were seen among younger woman -- with rates almost five times higher in women in their 40s than women in their 30s, he writes in the editorial, and adds that women in their late 30s and 40s should perhaps discuss non-hormonal contraceptive options with their physician.
Hunter also points out that hormonal contraceptives, namely oral contraceptives, are linked to a lower risk of ovarian, endometrial and colorectal cancers later in life.
"The benefits (against these other cancers) persists for one to two decades," he said, while breast cancer risk declines more rapidly. "Overall, it may be more beneficial."
But the search must continue for risk-free options, he concluded. "A lot of momentum has gone," he said. "But this study reminds us this is an important objective."
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