TUESDAY, June 29 (HealthDay News) -- Slightly less than half of early-stage breast cancer patients complete their full prescribed course of hormone therapy, finds a new study.

U.S. researchers examined the automated pharmacy records of 8,769 women diagnosed with stage 1, 2 or 3 hormone-sensitive breast cancer between 1996 and 2007. Each woman filled at least one prescription for hormone therapy within a year of breast cancer diagnosis. The women used tamoxifen (43 percent), aromatase inhibitors (26 percent) or both (30 percent).

Overall, only about 49 percent of the women completed their full prescribed regimen of hormone therapy, the study found. After 4.5 years, 32 percent of the women had stopped taking their hormone therapy. Of those who did not stop, 72 percent finished on schedule (meaning they took their medication more than 80 percent of the time).

Those most likely to discontinue hormone therapy early were found to be women younger than 40. Among these women and women older than 75, those who had a lumpectomy rather than a mastectomy and those who had other medical illnesses were more likely to discontinue the therapy. Those most likely to complete 4.5 years of hormone therapy were Asian/Pacific Islanders, women who'd undergone chemotherapy in the past, those who were married and women who had longer prescription refill intervals.

In general, the researchers said, women stop hormone therapy early for a variety of reasons, including such side effects such as joint pain, hot flashes and fatigue; a lack of understanding about the benefits of the therapy; and the high cost of medications and insurance co-payments.

"Physicians are often unaware of patient compliance, and this is becoming an increasingly important issue in cancer," the study's leader, Dr. Dawn Hershman, an associate professor of medicine and epidemiology at Columbia University Medical Center, said in a news release from the Journal of Clinical Oncology. The study appears online June 28 in the journal.

"It's very disturbing that patients under 40 had the highest discontinuation rates because those patients have the longest life expectancy," Hershman said. "If we can better understand the issues surrounding compliance with hormonal therapy, this might help us understand why patients don't adhere to other treatments that are moving out of the clinic and into the home, such as oral chemotherapy, as often as we would like."

More information

The American Cancer Society has more about hormone therapy for breast cancer.