Amarin Corp. plunged in premarket trading today after an FDA decision cast more doubt on the future of the specialty drugmaker’s lead product.

The Irish company said Tuesday after markets closed that the Food and Drug Administration no longer considers a change in triglyceride levels enough to establish the effectiveness of a drug that aims to reduce the risk of cardiovascular problems like heart attacks in certain patients.

Amarin has developed Vascepa, a prescription-strength form of an omega-3 fatty acid found in fish oil. Regulators approved the drug in 2012 for a relatively narrow use in patients with unusually high triglyceride levels.

The drugmaker has also asked the FDA to allow marketing of the drug for an expanded use in patients with high triglyceride levels and heart disease who are already taking a statin drug to help control their cholesterol.

The FDA told Amarin that clinical research results do not support the hypothesis that a triglyceride-lowering drug significantly reduces the risk for cardiovascular problems in these patients.

A panel of outside advisers to the FDA had already voted earlier this month against recommending the broader use.

Panelists said that while Vascepa significantly lowers fat levels, it is unclear whether that actually translates into fewer heart attacks. The panel members said the FDA should delay a decision on expanding the drug’s use until Amarin completes a study of heart attack rates in patients. Results from that study are not expected until 2015 or 2016.

Amarin shares have plunged since the panel decision was announced. The stock dropped another 16 percent, or 35 cents, to $1.75 Wednesday in premarket trading. That put the share price down about 78 percent since the stock closed 2012 at $8.09.