The US Supreme Court on Tuesday takes up a case examining whether a Vermont law that blocks access to prescription-drug records violates the free-speech rights of pharmaceutical companies.

At issue is Vermont’s 2007 Prescription Confidentiality Law. The measure bars drug companies from obtaining and using a doctor’s prescription records for marketing purposes unless the doctor grants prior permission for companies to use the data.

The prescription-drug data do not identify patients. But drug stores, which maintain the records, can sell them to data-mining companies, which then sell the information to pharmaceutical companies.

The information is useful because drug companies can use it to identify doctors who routinely prescribe cheaper generic drugs, the Vermont legislature concluded. Those doctors are then targeted through promotions to switch to drugs that tend to be more expensive and sometimes carry a more serious risk of side effects, the lawmakers found.

Three data-mining companies and the trade group Pharmaceutical Research and Manufacturing of America sued, claiming the law violated their free-speech right by imposing restrictions on a form of commercial speech.

A federal judge upheld the law, but an appeals court ruled 2 to 1 that it violated First Amendment protections.

Urging the high court to uphold the state law, Assistant Vermont Attorney General Bridget Asay said the measure does nothing more than require a doctor’s consent before allowing the doctor’s nonpublic prescribing information to be sold by pharmacies and used for marketing.

She said the state law was similar to statutes that help consumers avoid unwanted junk mail or obnoxious telemarketing sales pitches at dinner time.

“Vermont’s law allows doctors, not the government, to decide whether their nonpublic prescription information should be sold to pharmaceutical manufacturers for use as a marketing tool,” Ms. Asay wrote in her brief.

“Allowing doctors to control the commercial use of their prescribing information is consistent with First Amendment values,” she said.

The industry trade group, PHARMA, has a different perspective on the case.

“Vermont’s law … discriminates against the speech of pharmaceutical manufacturers,” wrote Lisa Blatt in her brief urging the court to overturn the law.

“The law bans manufacturers from speaking to physicians based on prescriber data unless the doctor has previously authorized such use on his licensing form,” Ms. Blatt said. “No such ban applies to insurance companies and other market participants that use the same data to convey their views about medicines to physicians.”

“The First Amendment bars the government from picking sides by restricting speech,” she said.

Twenty-six friend of the court briefs have been filed in the case. Among the nine filed supporting the Vermont law are briefs by the Obama administration, 35 states and the District of Columbia, the New England Journal of Medicine, and various state medical associations.

Seventeen friend of the court briefs are urging the justices to strike down the Vermont law. They were filed on behalf of various business and research groups and a collection of media companies.

The case is Sorrell v. IMS Health Inc. A decision is expected by the end of June.