The Supreme Court ruled Thursday that states cannot limit drug manufacturers' use of information about which prescription drugs doctors like to prescribe.

The court voted 6-3 to strike down a Vermont data-mining law aimed at boosting the use of generic drugs by controlling the flow of information about brand-name medications. The ruling imperils similar laws in Maine and New Hampshire.

The laws prevent the sale of information about individual doctors' prescribing records without the doctors' permission.

Pharmacies get that information when they fill prescriptions. They sell the information, without patient names, to data mining companies that, in turn, provide drug makers with a detailed look at what drugs doctors choose for their patients.

Drug company sales representatives can use the information to tailor their pitch to individual doctors.

Justice Anthony Kennedy said in his majority opinion that the Vermont law violates the speech rights of the data-mining and pharmaceutical companies.

Kennedy said that law grew out of a desire to control health care costs by increasing the use of generic drugs. But he said, "the state cannot engage in content-based discrimination to advance its own side of a debate."

In dissent, Justice Stephen Breyer said the law should have been upheld as a constitutional regulation of business activity. Justices Ruth Bader Ginsburg and Elena Kagan also signed on to Breyer's dissent.