Some birth control pills can raise breast cancer risk in women, a new study suggests.

Oral contraceptives, linked to breast cancer risk, have high levels of estrogen and few other chemicals, researchers said.

The study was conducted by scientists at the Fred Hutchinson Cancer Research Center in Seattle and colleagues. Other kinds of birth control pills that have lower levels of estrogen didn't carry breast cancer risk.

The study was based on data from 1,102 women who were diagnosed with breast cancer and 21,952 controls. Researchers found that women who used oral contraceptives in the last one year had 50 percent higher risk of developing breast cancer as compared to women who had never or used them long ago.

All the participants in the study were from the Group Health Cooperative in the Seattle-Puget Sound area.

"Our results suggest that use of contemporary oral contraceptives [birth control pills] in the past year is associated with an increased breast cancer risk relative to never or former oral contraceptive use, and that this risk may vary by oral contraceptive formulation," said Elisabeth F. Beaber, PhD, MPH, a staff scientist in the Public Health Sciences Division of Fred Hutchinson Cancer Research Center in Seattle, Washington.

The idea that oral contraceptives might raise breast cancer risk has been around for quite some time now. A study conducted in 1996 had found that women who use birth control pills have a slightly increased risk of developing breast cancer.

In the study, birth control pills containing high levels of estrogen raised breast cancer risk by 2.7-fold, while those containing moderate levels of the hormone increased risk 1.6 times. Also, oral contraceptives with ethynodiol diacetate raised cancer risk 2.6-fold, while triphasic combination pills with 0.75 milligrams of norethindrone increased risk of breast cancer 3.1-fold, researchers said.

Beaber maintains that genetic factors such as mutations in the BRCA genes are more strongly linked to overall breast cancer risk than contraceptive use. The latest study didn't account for genetic basis of cancers.

The absolute risk of breast cancer in women (on oral contraceptives) under 50 years of age is "less than 2 percent," Beaber told MedPage.

"Our results require confirmation and should be interpreted cautiously," added Beaber in a news release. "Breast cancer is rare among young women and there are numerous established health benefits associated with oral contraceptive use that must be considered. In addition, prior studies suggest that the increased risk associated with recent oral contraceptive use declines after stopping oral contraceptives."

The study is published in the journal Cancer Research.