The warning applies only to women with a family history of breast cancer and not to the general population, where studies have shown a slight increase in breast cancer risk from birth control pills but a diminished risk 10 years after use of the pills is stopped. Fewer than 10 percent of the 180,000 cases a year of breast cancer in the United States are thought to be familial, or inherited. It is not known whether the findings apply to women who took birth control pills after 1975, because earlier pills contained far higher levels of estrogen, which is believed to spur the growth of some breast cancers.
Newer pills contain lower doses of the hormone.
The authors of the study say it is the first to look for a link between birth control pills and breast cancer risk in families with inherited breast cancer. The researchers followed more than 6,000 sisters, daughters, nieces, granddaughters and other female relatives of 426 women who were found to have breast cancer from 1944 to 1952. The relatives were interviewed from 1991 to 1996. The study, by doctors at the Mayo Clinic in Rochester, Minn., and the University of Minnesota in Minneapolis, is being published today in the Journal of the American Medical Association. The researchers found that in women who already had a higher than average risk of breast cancer because of family history, early versions of the pill made the risk even worse.