Dr. Bevers suggests the drugs for women whose lifetime odds exceeds 20 percent. That could include, for example, a 55-year-old woman who began menstruating early (increasing the risk), had her first child late (again increasing the risk), and whose mother and sister got breast cancer. About half the time, though, women with that kind of risk turn down the drugs, Dr. Bevers said. “The No. 1 reason I hear is, ‘Oh, I just don’t like to take medications,’ ” she added.

Others, like Cecilia Anderson, who is 57 and lives in Houston, worry about side effects. “I felt like my quality of life was in question,” she said. “I am busy, I am out there. I totally love my life and don’t want it to be compromised.” Her lifetime risk of breast cancer is 20.5 percent, compared with an average risk of 9.8 percent for a woman her age. Ms. Anderson declined the drugs. “I live a different lifestyle,” she said. “I eat organic foods, I exercise. Through all of that comes a spiritual element as well. Mind, body, and spirit are all connected.”

Studies’ Complications

Then came the studies of finasteride and dutasteride for prostate cancer. The drugs block the conversion of testosterone to dihydrotestosterone, a hormone that prostate cancers need to grow. They are on the market to shrink the prostate in older men, whose prostates often enlarge. (Finasteride is also sold to grow hair  but the dose is one-fifth the dose that shrinks prostates and that dose has not been tested for cancer prevention.) Doctors can prescribe the drugs for cancer prevention but, at this point, that is not on their label.

The prostate cancer studies were complicated by other another factor; at first, researchers thought, erroneously, that finasteride was actually spurring the growth of aggressive prostate cancers. The drug’s side effects can include impotence or decreased ejaculate. But the Food and Drug Administration concluded that these effects, if they occur at all, are gone after a year.

Now, even though the F.D.A. deemed the drug’s adverse reactions to be “usually mild and transient.” The American Urological Association and the American Society for Clinical Oncology recommend that men 50 and older consider taking it. But there appears to be little interest even among high-risk men.

Dr. Vogel wonders what message the indifference to the breast cancer and prostate cancer drugs is sending. Why would a company want to develop drugs for cancer prevention?

That is a lesson that hits home, said Dr. Gregory Curt, an oncologist who directs emerging products at AstraZeneca, a drug company.