WASHINGTON — The Supreme Court on Thursday handed drug companies two significant victories, one limiting suits from people injured by generic drugs and the other striking down a law that banned some commercial uses of prescription data.

In the first case, Pliva v. Mensing, No. 09-993, the court split 5 to 4 along ideological lines in ruling that the makers of generic drugs — which account for 75 percent of prescriptions dispensed nationwide — may not be sued under state law for failing to warn customers about the risks associated with their products.

Two years ago, in Wyeth v. Levine, the court decided the same question in the context of brand-name drugs but came to the opposite conclusion. That decision was based in large part on the fact that such drug companies can sometimes change the labels on their products without permission from the Food and Drug Administration.

Justice Clarence Thomas, writing for the majority on Thursday, acknowledged that in the eyes of injured consumers, the new distinction between generic and brand-name drugs “makes little sense.” But he said it followed from the way the two kinds of companies are treated under federal law.

The manufacturers of generic drugs, he said, must use the same warning labels as the corresponding brand-name drugs, and they may not unilaterally alter those labels. That means, Justice Thomas wrote, that makers of generic drugs are caught in an impossible bind: they can comply with a state law requiring them to change their labels or the federal law prohibiting changes, but not both.

Given that impossibility, federal law pre-empts state law under the Constitution’s supremacy clause, he wrote.

In dissent, Justice Sonia Sotomayor said the majority opinion invented “new principles of pre-emption law out of the air” and will lead to “absurd consequences.”

“As the majority itself admits,” Justice Sotomayor wrote, “a drug consumer’s right to compensation for inadequate warnings now turns on the happenstance of whether her pharmacist filled her prescription with a brand-name drug or a generic.”

The decision considered three consolidated cases brought by women who took generic metoclopramide, which is sold under the brand name Reglan. They took the drug for stomach ailments and developed a serious neurological disorder. Appeals courts ruled against the drug makers, saying that the federal regulatory regime did not block claims under state law.

The Supreme Court reversed those decisions on Thursday, rejecting what Justice Thomas called the “fair argument,” that the defendants should have at least tried to persuade the federal drug agency to let them use a safer label.

But the process of asking the agency to change a label, he wrote, can be as complicated as a children’s board game.

“If they had done so,” Justice Thomas wrote of a possible request for a label change, “and if the F.D.A. decided there was sufficient supporting information, and if the F.D.A. undertook negotiations with the brand-name manufacturer, and if adequate label changes were decided on and implemented, then the manufacturers would have started a Mouse Trap game that eventually led to a better label on generic metoclopramide.”

Chief Justice John G. Roberts Jr. and Justices Antonin Scalia, Anthony M. Kennedy and Samuel A. Alito Jr. voted with the majority.

In her dissent, Justice Sotomayor wrote that she agreed that the makers of generic drugs could not unilaterally change their labels. But she said that did not allow them to remain idle after learning of safety issues.

“Had the manufacturers invoked the available mechanism for initiating label changes,” she wrote, “they may well have been able to change their labels in sufficient time to warn” the women injured by their drugs.

The majority opinion, Justice Sotomayor said, may reduce the demand for generic drugs and put doctors in an ethical bind.

Justice Ruth Bader Ginsburg, Stephen G. Breyer and Elena Kagan joined the dissent.

In a second decision on Thursday, Sorrell v. IMS Health, No. 10-779, a six-justice majority of the court struck down a Vermont law that banned some but not all uses of prescription information collected by pharmacies.

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