The warning from Johnson and Johnson subsidiary Ortho McNeil, makers of Ortho Evra, says women using the patch will be exposed to about 60 percent more estrogen than those using typical birth control pills because hormones from patches get into the bloodstream and are removed from the body differently than those from pills.

Thursday's warning comes four months after The Associated Press reported in July that patch users die and suffer blood clots at a rate three times higher than women taking the pill.

Ortho McNeil spokesman Michael Beckerich was not immediately available for comment about Thursday's warning, which was approved by the Food and Drug Administration.

More than 4 million women have used the patch since it went on sale in 2002. Several lawsuits have been filed by families of women who died or suffered blood clots while using the patch, and lawyers said more are planned.

Documents released to attorneys as a result of that litigation show Ortho McNeil has been analyzing the FDA's death and injury reports, creating its own charts that document a higher rate of blood clots and deaths in association with the patch than with the pill.

In addition, an internal Ortho McNeil memo shows that the company refused, in 2003, to fund a study comparing its Ortho Evra patch to its Ortho-Cyclen pill because of concerns there was "too high a chance that study may not produce a positive result for Evra" and there was a "risk that Ortho Evra may be the same or worse than Ortho-Cyclen."

Last week, in response to AP questions about the Ortho McNeil memo, Beckerich said in a written statement that "decisions to fund studies are based upon scientific merit."

Beckerich said Ortho McNeil is conducting its own epidemiological study "designed with input from the FDA and similar to those previously conducted with the Pill."

Although the patch and most birth-control pills contain the same amounts of estrogen, new published studies show that women using the patch absorb about 50 percent more estrogen than with the pill, said Dr. Leslie Miller, an associate professor of obstetrics and gynecology at the University of Washington.

When women take the pill, the medication is absorbed into the bloodstream through the digestive tract. In the process, about half of the estrogen dose is lost.

Hormone levels in women on the pill are highest one or two hours after taking it, Miller said. Twelve hours later, estrogen levels are quite low, meaning the body is not exposed to high levels of estrogen 24 hours a day.

But the patch causes higher estrogen levels since delivery of medication continues all day. Those elevated levels but may be high enough to increase some women's risk of blood clots, Miller said.

"If the patch is delivering too much estrogen, then it may need to be redesigned," Miller said. "Women should not just take off their patch, they risk pregnancy. If they are worried and want to change off the patch they can wait to get something else."

Jennifer Cowperthwaite, 26, of Broad Brook, Conn., still suffers breathing problems after a blood clot reached her lungs two years ago while using the patch. She said the warnings were long overdue.

"I wish I had known," she said. "It's quite likely I would never have used it."

Erika Klein's sister Kathleen Thoren died a year ago from blood clots in her brain that the coroner said were brought on by Ortho Evra. She said women deserve to be informed when making birth control decisions.

"Women have a right to know the true risks and make their decisions based on that information," she said. "No one should have to go through what my sister went through