But it said the results are preliminary and do not require immediate action other than advising women to discuss the risk with their doctor.

The results of the study, and another that found no increased risk, were made public Thursday by the patch's manufacturer.

"The results are preliminary and further evaluation is necessary to understand what these results mean," Dr. Daniel Shames, director of the division of reproductive and urological drug products at US Food and Drug Administration, said at a briefing.

'Fairly Unusual Events'

The finding comes from one of two studies comparing the patch and pill, said Ortho Women's Health & Urology, maker of the once-a-week patch. The company is owned by Johnson & Johnson.

Last year, an investigation by The Associated Press, citing federal death and injury reports, found higher rates of blood clots in women using the patch.

The first study found no increased risk of clots but the interim results from the second study suggested a twofold increase in the risk of venous thromboembolic events, or clots in the legs and lungs, in women using the patch, Ortho said.

However, because the confidence intervals of the results for the two forms of contraceptive overlap, there actually may be no increased risk from the patch or it may be more than twice, Shames said at a briefing.

He said the risk of a nonfatal blood clot is about one per year in 10,000 women not using a contraceptive. For those using a hormonal contraceptive such as the patch or pill the risk rises to between three and five, he said.

"These are fairly unusual events," said Shames. He noted that in preapproval testing of the patch on about 3,000 women there were two reports of blood clots, but one involved a woman who had had surgery.

60 Percent More Estrogen

The ongoing studies also are looking at the risk of heart attacks and strokes among users of the two types of contraception. Currently there is no difference but the numbers are small and it will take another 18 months to see if a difference occurs, Shames said.

The company said that the risk of clots remains rare and that they have been reported as a potential risk of all hormonal contraceptives.

Release of the interim results comes four months after the FDA warned women that the increased levels of hormones released by the patch put them at higher risk of blood clots and other serious side effects. Ortho said it shared the results of the latest studies with the FDA.

Additions to the patch label made in November warned women that they would be exposed to about 60 percent more estrogen than those who use birth-control pills.

Since the patch went on sale in 2002, more than 4 million women have used it.

The investigation by The Associated Press found that patch users die and suffer blood clots at a rate three times higher than women taking the pill. About a dozen women died in 2004 from blood clots believed linked to use of the patch, the AP reported. Dozens more suffered strokes and other clot-linked problems.

Health officials warn that women who smoke should not use the patch, since smoking increases the risk of stroke and heart attack.

Associated Press Writer Andrew Bridges contributed to this report from St. Louis.