For years, Johnson & Johnson obscured evidence that its popular Ortho Evra birth control patch delivered much more estrogen than standard birth control pills, potentially increasing the risk of blood clots and strokes, according to internal company documents.

But because the Food and Drug Administration approved the patch, the company is arguing in court that it cannot be sued by women who claim that they were injured by the product  even though its old label inaccurately described the amount of estrogen it released.

This legal argument is called pre-emption. After decades of being dismissed by courts, the tactic now appears to be on the verge of success, lawyers for plaintiffs and drug companies say.

The Bush administration has argued strongly in favor of the doctrine, which holds that the F.D.A. is the only agency with enough expertise to regulate drug makers and that its decisions should not be second-guessed by courts. The Supreme Court is to rule on a case next term that could make pre-emption a legal standard for drug cases. The court already ruled in February that many suits against the makers of medical devices like pacemakers are pre-empted.