WASHINGTON - Federal regulators are working on a stronger label for a widely used diabetes drug marketed by Amylin Pharmaceuticals Inc. and Eli Lilly & Co. after deaths were reported with the medication despite earlier government warnings.

The Food and Drug Administration said Monday it has received six new reports of patients developing a dangerous form of pancreatitis while taking Byetta. Two of the patients died and four were recovering.

Regulators stressed that patients should stop taking Byetta immediately if they develop signs of acute pancreatitis, a swelling of the pancreas that can cause nausea, vomiting and abdominal pain. The FDA warned that it is very difficult to distinguish acute pancreatitis from less dangerous forms of the condition.

The FDA announcement updated an October alert about 30 reports of Byetta patients developing pancreas problems. None of those cases were fatal, but Byetta’s makers agreed to add information about the reports to the drug’s label.

However, the FDA made clear Monday that it is seeking a stronger, more prominent warning about the risks.

Amylin and Eli Lilly said in a statement that patients taking Byetta have shown “very rare case reports of pancreatitis with complications or with a fatal outcome.” The companies added that diabetes patients are already at increased risk of pancreatitis compared with healthy patients. The pancreas produces several important biological fluids, including insulin — the sugar-regulating hormone that most diabetics lack.

The FDA said doctors should consider prescribing other medications to patients with a history of pancreas problems.

Byetta competes against blockbuster drugs from GlaxoSmithKline PLC and Takeda Pharmaceuticals in the $24 billion global market for diabetes medications, according to health care research firm IMS Health.

More than 700,000 patients with type 2 diabetes have used the injectable drug since it was launched in June 2005. It is jointly developed and manufactured by San Diego-based Amylin and Eli Lilly.

Byetta’s $636 million in sales made up about 80 percent of Amylin’s total revenue last year. The drug accounted for just 3 percent of Eli Lilly’s sales. The companies are developing a long-lasting version of Byetta that could be injected once weekly, instead of twice daily.

Robert Baird & Co. analyst Thomas Russo cautioned investors that concerns about pancreatitis could affect the FDA’s review and requirements for the new version. Russo rates Amylin “outperform,” but said in a note to investors he could revise that rating as more information becomes available.

Shares of Amylin Pharmaceuticals fell $4.45, or 13 percent, to $29.76 Monday, while Indianapolis-based Eli Lilly dipped 73 cents to $48.05.