But attitudes are changing as a result of a flurry of research and new attention to the after-effects of life-saving treatment. There is now widespread acknowledgment that patients with cognitive symptoms are not imagining things, and a growing number of oncologists are rushing to offer remedies, including stimulants commonly used for attention-deficit disorder and acupuncture.

“Until recently, oncologists would discount it, trivialize it, make patients feel it was all in their heads,” said Dr. Daniel Silverman, a cancer researcher at the University of California, Los Angeles, who studies the cognitive side effects of chemotherapy. “Now there’s enough literature, even if it’s controversial, that not mentioning it as a possibility is either ignorant or an evasion of professional duty.”

That shift matters to patients.

“Chemo brain is part of the language now, and just to have it acknowledged makes a difference,” said Anne Grant, 57, who owns a picture-framing business in New York City. Ms. Grant, who had high-dose chemotherapy and a bone marrow transplant in 1995, said she could not concentrate well enough to read, garbled her sentences and struggled with simple decisions like which socks to wear.

Virtually all cancer survivors who have had toxic treatments like chemotherapy experience short-term memory loss and difficulty concentrating during and shortly afterward, experts say. But a vast majority improve. About 15 percent, or roughly 360,000 of the nation’s 2.4 million female breast cancer survivors, the group that has dominated research on cognitive side effects, remain distracted years later, according to some experts. And nobody knows what distinguishes this 15 percent.

Image When she cant remember where she parked her car, Lu Ann Hudson uses a key fob that sets off a beep in it. Credit... Michael Houghton for The New York Times

Most oncologists agree that the culprits include very high doses of chemotherapy, like those in anticipation of a bone marrow transplant; the combination of chemotherapy and supplementary hormonal treatments, like tamoxifen or aromatase inhibitors that lower the amount of estrogen in women who have cancers fueled by female hormones; and early-onset cancer that catapults women in their 30s and 40s into menopause.