The drug, Femara, is expected to win Food and Drug Administration approval soon for women and have early breast cancer. It is already approved for treating advanced cases of the disease.
Femara and Arimidex, a similar drug already licensed for early breast cancer, are aromatase inhibitors, which block production of estrogen, a hormone that fuels the growth of most tumors that develop after menopause. Tamoxifen works differently, by blunting the ability of estrogen to enter cells.
A study reported earlier this year in Europe and published in Thursday's New England Journal of Medicine estimated that 84 percent of women given Femara versus 81 percent of those on tamoxifen would be alive without any signs of cancer five years after starting treatment.
The estimates were based on roughly two years of information on relapses among the 8,000 women in the study, done by researchers in the United States, Europe and Australia.
It was financed by Femara's maker, Novartis. Many of the researchers own stock in or are consultants for Novartis or companies with rival drugs.
Severalhave shown Femara or Arimidex to be better either as initial treatment or after a couple years of tamoxifen.
"These trials, with close to 30,000 participants, consistently demonstrate that treatment with an aromatase inhibitor alone or after tamoxifen treatment is beneficial," Dr. Sandra Swain of the National Cancer Institute wrote in an editorial in the journal.
The challenge now is figuring out how long women should take these drugs, which drug is best, and whether switching drugs at some point is helpful, she wrote.
Tamoxifen remains the top choice for women who get breast cancer before menopause because aromatase inhibitors aren't thought to be effective then.
Aromatase inhibitors do not raise the risk of blood clots or endometrial cancer as tamoxifen does, but they do increase the chances of bone problems such as osteoporosis. Women are often advised to take supplements or other medications to maintain bone density.
A third aromatase inhibitor, Pfizer Inc.'s Aromasin, also has shown promise for preventing recurrence when given after several years of tamoxifen. But it has not yet been tested against tamoxifen as initial therapy the way Femara and AstraZeneca PLC's Arimidex have been.
Each year, about 800,000 women around the world are diagnosed with early breast cancer and about three-fourths are of the type that might benefit from these drugs.