The warning applies only to women with a family history of breast cancer, not to the general population. Studies of the general population have shown a slight increase in breast-cancer risk from birth control pills, which disappears 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. Earlier pills contained far higher levels of estrogen, which is thought to spur the growth of some breast cancers.
Newer pills contain lower doses of the hormone.
The study, authors say, 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 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. Those who took the pill before 1975 and whose mothers or sisters had breast cancer had triple the risk of women with the same family history who did not take the pill.