It said women should be offered preventive drug treatment if they are estimated to have a 3 percent or greater risk of developing invasive breast cancer within five years and their risk of harm from the drugs is low. The estimate of risk is based on five factors: age; race/ethnicity; family history of breast cancer in a mother, sister or daughter; history of a breast biopsy; and breast density.

Dr. Karla Kerlikowske of the University of California at San Francisco, one of the developers of the calculator, said that although half of women have dense breasts, not all of them are high risk. Combining the degree of density with results from the calculator can help identify women who would benefit from more intensive screening measures as well as risk-reducing medication.

For women with a history of radiation therapy to the chest or preinvasive breast cancer, “chemoprevention is an important option,” Dr. Pace said. “For women who are at two to three times average risk of developing invasive cancer, the drugs can lower their risk to average.”

Unfortunately, the realities of modern life often run counter to factors known to reduce breast cancer risk, and not all known risk factors are modifiable, which makes it especially important for women to adopt those that are within their power to practice.

The main nonmodifiable characteristic is age, the biggest risk factor for this disease, and women today live nearly 30 years longer than they did a century ago. Puberty starts earlier, women have their first full-term pregnancies later, fewer than one mother in five breastfeeds for six months or longer, and some women enter menopause later than average, all of which increase the number of years that breasts are exposed to cancer-promoting hormones.

More women today drink alcohol and in greater amounts than in decades past, and breast cancer risk increases in direct proportion to how much and how often a woman drinks. Exposure has risen to environmental pollutants that may disrupt the normal balance of hormones and put breasts at risk. (However, neither drinking coffee nor using birth control pills adversely affects risk.)

Compounding the effects of these changes is the high rate of being overweight and obese in girls and adult women. Body fat produces estrogen and also stores environmental pollutants. The benefits of avoiding excess weight extend far beyond just cancer prevention.