A new prospective study has reported that current use of oral contraceptives (birth control pills) using levonorgestrel as the progestin component can triple the risk of breast cancer. The study was designed to investigate the association between current oral contraceptive use and risk of breast cancer. Previous studies reported an increased risk of breast cancer associated with use of oral contraceptives from the 1960s to 1980s. However, the relationship between today's oral contraceptive formulations (which have a lower hormone content) and risk of breast cancer risk has not been established. The study included 116,608 female nurses ages 25 to 42 years at enrollment in 1989. Lifetime oral contraceptive use and the specific formulations used was determined at baseline and the information was updated every two years. This data was related to risk of breast cancer through June 1, 2001.

A total of 1,344 cases of invasive breast cancer were diagnosed in the study group during the follow-up period. Past use of any type of birth control pill was not found to be associated with risk of breast cancer. Current use of any birth control pill was found to be associated with a marginally significant higher risk of breast cancer. When the results were examine by type of birth control pill, most of the additional risk was substantially accounted for by one specific birth control formulation: triphasic preparations with levonorgestrel as the progestin. Women currently using this formulation had triple the risk of breast cancer compared to the overall study group. The authors conclude that current use of oral contraceptives carries an excess risk of breast cancer, much of which may be accounted for by levonorgestrel used in triphasic preparations. Since different oral contraceptive formulations appear to convey different risks of breast cancer; ongoing monitoring of these associations is necessary as oral contraceptive formulations change.