May 7 (Reuters) - Amarin Corp Plc filed suit against the U.S. Food and Drug Administration on Thursday for restricting its right to promote its fish oil drug Vascepa for an unapproved, or off-label, use.

The suit, filed on Thursday in U.S. District Court for the Southern District of New York, argues that the FDA's ban violates Amarin's right to free speech under the First Amendment of the U.S. Constitution.

The FDA's rule "severely restricts medical professionals' access to information from the source most knowledgeable about the drugs: the drug manufacturers," the suit says.

It is the first lawsuit of its kind to be launched against the FDA since 2012, when the 2nd Circuit Court of Appeals overturned the conviction of a drugs salesman, Alfred Caronia, caught talking to physicians about off-label uses of the narcolepsy drug Xyrem.

The court ruled that the First Amendment protected truthful and non-misleading off-label speech.

(Reporting by Toni Clarke in Washington; Editing by Will Dunham)