The Supreme Court on Thursday ruled that makers of generic drugs cannot be sued for failing to warn consumers of the possible side effects of their products if they copy the exact warnings on the medicines' brand-name equivalents.

The majority opinion acknowledged that the decision dealt an "unfortunate hand" that "makes little sense" to those who were harmed by generic drugs and are unable to sue the drugmakers.

A sharply divided court decided in a 5-4 judgment that the federal law requiring generics to have the same warnings as their brand-name equivalents trumps state laws that allow people to sue drug companies that fail to present proper warnings for their products.

This decision could affect millions of Americans, given that generics make up 75 percent of the prescription drugs dispensed in the United States, justices said.

"As a result of today's decision, whether a consumer harmed by inadequate warnings can obtain relief turns solely on the happenstance of whether her pharmacist filled her prescription with a brand-name or generic drug," Justice Sonia Sotomayor said in a strongly worded dissent. "The court gets one thing right: This outcome makes little sense."

Fordham Law School professor Benjamin Zipursky called the case especially striking, considering a "6-3 majority decided only two years ago in Wyeth v. Levine that non-generic manufacturers should not be shielded from failure-to-warn claims."

Gladys Mensing and Julie Demahy had sued PLIVA Inc. and other generic drug manufacturers in state courts in Minnesota and Louisiana over the labels for metoclopramide, the generic version of Reglan. The two women alleged that metoclopramide gave them a severe neurological movement disorder called tardive dyskinesia, but none of the generic drug's manufacturers and distributors made any effort to include warnings on the label.

PLIVA Inc. and other generic drug manufacturers argue that it's not their legal responsibility to request label changes, and that lawsuits filed in state courts over the content of labels improperly infringe on the Food and Drug Administration's enforcement area.

Reglan did not have a warning about tardive dyskinesia when Mensing and Demahy were taking it. The FDA ordered warnings about tardive dyskinesia to be added to Reglan and metoclopramide in February 2009.

Justice Clarence Thomas, who wrote the majority opinion, said it was impossible for generic drug manufacturers to comply both with federal laws requiring them to have the same label as the brand-name drugs and state laws that require them to add additional warnings. So the federal law prevails.

Thomas acknowledged the "unfortunate hand" dealt to people who are harmed by inadequate labels on generic drugs. They would be able to sue the companies who make brand name drugs for the same offense but are barred from suing the generic manufacturers.

For those people, the pre-emption of federal law over state laws makes little sense, Thomas said. "Had Mensing and Demahy taken Reglan, the brand-name drug prescribed by their doctors ... their lawsuits would not be pre-empted. But because pharmacists, acting in full accord with state law, substituted generic metoclopramide instead, federal law pre-empts these lawsuits. We acknowledge the unfortunate hand that federal drug regulation has dealt Mensing, Demahy and others similarly situated."

He was joined in his judgment by Chief Justice John Roberts and Justices Antonin Scalia, Anthony Kennedy and Samuel Alito.

Lawyer Jay Lefkowitz, who represented Teva Pharmaceuticals in this case, said the Supreme Court "hit the nail on the head today."

"Federal law does not permit states to hold generic drug manufacturers liable for using the very warnings federal law required them to use," he said.

Sotomayor, who wrote the dissent, said PLIVA and the other generic manufacturers didn't even try to get the Food and Drug Administration to change the labels of Reglan. If they had, the FDA might have forced Reglan to change its label, she said.

"Once that occurs, a generic manufacturer is in full compliance with both federal law and a state-law duty to warn," she said.

The high court's decision overturns the ruling by the 8th U.S. Circuit Court of Appeals in St. Louis, which agreed with Sotomayor that more should have been done to warn consumers about possible risks.

The cases are PLIVA Inc. v Mensing, 09-993, Actavis Elizabeth LLC v. Mensing, 09-1039 and Activis, Inc., v. Demahy, 09-1501.