SAN ANTONIO ? A newer drug prevented far more breast cancers from recurring in older women than the old standby tamoxifen and with far fewer side effects, doctors said Wednesday, citing a new study.

Cancer specialists said Arimidex is likely to become the first-choice treatment for most women who have had the disease, and they predicted a wider role for similar drugs of its type, called aromatase inhibitors.

New research suggests Arimidex might be able to prevent 70 percent to 80 percent of the most common type of tumors that occur in women after menopause, compared with the 50 percent that tamoxifen is credited with warding off.

Women with early-stage breast cancer who took Arimidex for five years were less likely to have cancer recur, develop in the other breast, or spread throughout their bodies than women who took tamoxifen.

“Arimidex is a more effective treatment. This is a better drug,” said Dr. Aman Buzdar, a specialist at the University of Texas’ M.D. Anderson Cancer Center. He headed the U.S. portion of the study, which involved about 6,000 women in 21 countries.

The studies were largely funded by the makers of the newer drugs, but experts not involved in the work were impressed by the results.

“The big message is, women will have a better outcome if they take aromatase inhibitors,” said Dr. Paul Goss of Massachusetts General Hospital, an expert on the drugs.

None of the studies change tamoxifen’s status as the drug of choice for women who get breast cancer before menopause, because the newer drugs aren’t thought to be effective then.

Tamoxifen revolutionized breast cancer treatment when it came into use some three decades ago and studies showed it could cut recurrence risk in half. It blunts the effects of estrogen, a hormone that promotes the growth of about three-fourths of the tumors in postmenopausal women.

Aromatase inhibitors prevent estrogen from being made in the first place, and don’t raise the risk of blood clots and endometrial cancer as tamoxifen does. Three are available: AstraZeneca PLC’s Arimidex, Pfizer Inc.’s Aromasin and Novartis Pharmaceuticals’ Femara.