The shares of Amarin Corp. plc, which has research-and-development headquarters in Groton, fell sharply Tuesday after the company disclosed that regulators have not yet decided whether to grant the maximum five-year protection from generic competition for its heart drug Vascepa.

Drugs like Vascepa that are designated as new chemical entities usually enjoy a five-year period of protection, so the lack of quick action by the U.S. Food and Drug Administration led to speculation that regulators might opt for three-year exclusivity.

The stock finished the day down about 6 percent, and analysts predicted more volatility until the FDA ruling is made public.