Tamoxifen, the pill that prevents breast cancer in high-risk women, does not appear in the long run to save many lives, U.S. researchers reported on Monday.

Women at the highest risk of breast cancer do appear to live longer if they take tamoxifen, the researchers report in the latest issue of the journal Cancer.

But for women at the low end of the high risk group, the sometimes serious side effects of tamoxifen outweigh the benefits, Dr. Joy Melnikow of the University of California, Davis, and colleagues reported.

Tamoxifen can cause blood clots and uterine cancer.

"We found that for women at the lower end of the high-risk range for developing breast cancer, there is a very small likelihood that taking tamoxifen will reduce mortality," Melnikow said in a statement.

Melnikow and her colleagues calculated that tamoxifen can extend life expectancy only when a woman's five-year risk of developing breast cancer is 3 percent or higher. This is especially true for women who have not had a hysterectomy, and thus risk endometrial cancer from taking tamoxifen.

Many women are in any case switching to a newer class of drugs known as aromatase inhibitors to treat breast cancer or to the osteoporosis drug raloxifene to prevent it.

Raloxifene, made by Eli Lilly and Co. under the name Evista, has been shown to prevent breast cancer as well as tamoxifen does, without causing as many blood clots, cataracts or as many cases of uterine cancer.

In June, researchers reported that women with breast cancer who switched to Pfizer Inc.'s drug Aromasin after taking tamoxifen were 17 percent less likely to die.

Tamoxifen blocks estrogen, which can help fuel the growth of tumors in some cases.

In women considered at high risk of breast cancer, usually meaning they have a close relative with breast cancer, have had several suspicious-looking lumps, or other conditions, tamoxifen reduced their risk of breast cancer by 49 percent.

Aromasin, known generically as exemestane, and similar drugs inhibit the enzyme aromatase, which is needed to produce estrogen. The aromatase inhibitors are now being used just after breast cancer surgery instead of tamoxifen in many women to keep the disease from returning.

They are not approved for prevention of breast cancer.

Tamoxifen was sold by AstraZeneca Plc under the name Nolvadex but is now marketed by several generic drug makers. It remains the only drug approved for use in preventing breast cancer in women who have not yet reached menopause.

Breast cancer is the second leading cause of cancer death among U.S. women, after lung cancer. More than 200,000 people are diagnosed and another roughly 40,000 die from it each year, according to the American Cancer Society.