MedWire News: Current use of oral contraceptives is associated with a moderately increased risk for breast cancer, which is mainly attributed to levonorgestrel used in triphasic preparations, show results from the Nurses Health study II.
Past use of levonorgestrel-containing triphasic contraceptives and current and past use of other formulations was not associated with a significantly increased breast cancer risk, the researchers remark.
"Previous studies convincingly showed an increase in risk for breast cancer associated with current or recent use of oral contraceptives from the 1960s to 1980s," note David Hunter (Harvard Medical School, Boston, Massachusetts, USA) and colleagues.
To assess the association of contemporary oral contraceptive formulations with breast cancer risk, Hunter and team analyzed data from 116,608 female nurses aged 25-42 years at study enrollment in 1989. Information on contraceptive use was subsequently updated every 2 years and the risk for breast cancer was examined up to June 2001.
The researchers report that during 1,246,967 person-years of follow-up, 1,344 cases of invasive breast cancer were diagnosed.
Past use of any oral contraceptive was not significantly related to breast cancer risk (relative risk=1.12], whereas current use of any oral contraceptive was related to a marginally significant 1.33-fold increased risk for breast cancer.
When the researchers analyzed the data according to the type of progestin formulation used, they found that current users of triphasic ethinyl estradiol combined with levonorgestrel - commonly prescribed in the 1990s - had a significant 3.05-fold increased risk for breast cancer, compared with women who had never used oral contraceptives.
The age-standardized incidence of breast cancer among never-users of oral contraceptives was 98 per 100,000 person-years compared with 227 per 100,000 person-years among current users of the triphasic ethinyl estradiol plus levonorgestrel formulation.
Of note, the most commonly used triphasic formulation contains norethindrone as the progestin rather than levonorgestrel, and this was not associated with an increased breast cancer risk. There was also no increased risk for levonorgestrel-containing nontriphasic formulations.
Hunter and co-authors conclude that their findings suggest "different oral contraceptive formulations might convey different risks of breast cancer."
"Ongoing monitoring of these associations is necessary as oral contraceptive formulations change," they add in the journal Cancer Epidemiology Biomarkers and Prevention.
MedWire (www.medwire-news.md) is an independent clinical news service provided by Current Medicine Group, a trading division of Springer Healthcare Limited. © Springer Healthcare Ltd; 2010
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