Drugmakers have long argued they should have the right to talk to doctors about unapproved uses for their products, as long as they are being truthful. And in some cases, courts have agreed. But the federal government still frowns on the practice and, in recent years, has fined drug companies billions of dollars for talking to doctors about so-called off-label uses for their medications.

On Thursday, Amarin Pharma took the unusual step of suing the Food and Drug Administration, arguing that it has a constitutional right to share certain information about its product with doctors, even though the agency did not permit the company to do so. Lawyers for the company said that they believed their case was the first time a manufacturer had pre-emptively sued the agency over the free-speech issue, before it had been accused of any wrongdoing. Other companies have sued the agency only after they have gotten into trouble.

“If you tell the truth — if you’re not misleading — then the First Amendment protects you when you provide this sort of information,” said Floyd Abrams, a noted First Amendment lawyer who is representing Amarin in the case, which was filed in United States District Court in New York. Amarin has its corporate headquarters in Dublin.

But others said that doing so sidesteps the authority of the F.D.A., which is responsible for making sure that drugs are safe and effective. “The First Amendment right is not an absolute right. It has limits. And it’s always been subject to a balancing test,” said Dr. Michael Carome, director of the health research group at Public Citizen, an advocacy group in Washington. “If this lawsuit were to succeed, it would be devastating for drug safety and undermine the drug approval process.”