LONDON (CNN) -- Cancer specialists in the United States are disputing results from two European studies that found no clear proof that the drug tamoxifen can prevent breast cancer, contradicting the promising results of an earlier U.S. study.

In its latest issue, the British medical journal The Lancet reports that researchers in Britain and Italy, who conducted separate studies involving a total of 7,900 women with a family history of breast cancer, found that the incidence of the disease was virtually the same in women who took tamoxifen as compared to those who didn't.

By contrast, in April, the U.S. National Cancer Institute, in a study involving nearly 13,400 women at high risk for breast cancer, found that women who took tamoxifen lowered their chances of developing the disease by 45 percent. The results were so promising that the institute ended the study early and offered tamoxifen to the women who were taking placebos.

"There are significant numbers of women requesting to take tamoxifen since the American study, and I have grave concerns about the widespread use of it in healthy women," said Dr. Trevor Powles, leader of the British study, which was conducted at Royal Marsden Hospital in London.

Powles said his study does not conclude that tamoxifen does not prevent breast cancer, only that there is insufficient evidence to say that it does. And tamoxifen can have significant side effects, including a higher risk of developing uterine cancer and blood clots in the lungs.

But two top U.S. specialists, Dr. Harmon Eyre, chief medical officer of the American Cancer Society, and Dr. Larry Norton of New York's Memorial-Sloan Kettering Hospital, told CNN that the results reported in The Lancet were misleading and incomplete -- and could cause confusion among women who might benefit from tamoxifen therapy.

Eyre and Norton said the British study, as well as the study at the European Institute of Oncology in Milan, Italy, were not long enough or large enough to generate complete information. And they said the women involved were at a low-to-moderate risk for breast cancer, which meant they were not as likely to benefit from tamoxifen as the women in the U.S. study, who were older and at greater risk for the disease.

Barnett Kramer, deputy director of the National Cancer Institute, said the new studies do not shake the institute's confidence in its findings.

"We are extremely confident, because of the number of women involved, that the data stands," he said. "The chance that our results occurred by chance were 1 in 10,000."

Tamoxifen blocks the effects of estrogen and retards growth of cancer cells that depend on the hormone. Scientists agree the drug fights the recurrence of breast cancer in women who have already developed the disease. The dispute arises over whether it can prevent cancer in healthy women.

The Associated Press contributed to this report.