U.S.-traded shares of Amarin Corp. PLC tumbled in premarket trading today, a day after the Irish drugmaker said it plans to sell 21.7 million American Depositary Shares in an underwritten public offering.

Amarin said it would sell the shares at market prices, and it gave the banks running the offering an option to buy an additional 3.3 million shares to cover excess demand. The company had about 150 million shares outstanding at the end of the first quarter. That total could grow to more than 175 million if the option is exercised in full.

Amarin said it will use proceeds from its offering to continue the commercial launch of Vascepa, its treatment for patients with high triglyceride levels.

Vascepa is a prescription form of fish oil designed to lower triglycerides, a type of fat in the bloodstream. The company started selling the drug in the United States in January for use with severely high levels of triglycerides. It also is seeking approval from the U.S. Food and Drug Administration to market Vascepa as a treatment for patients with high triglyceride levels who also are taking a statin therapy to control their cholesterol.

The FDA is expected to decide on that additional use by December.

Amarin also plans to use offering proceeds to prepare for the launch of Vascepa for that additional use.

Shares of Amarin fell 8.3 percent, or 51 cents, to $5.66 in premarket trading Tuesday. The stock still hasn’t recovered from a slump that started last December, when Amarin said it would hire its own sales force as it prepared for Vascepa’s launch. That disappointed investors who were hoping Amarin would be sold to a larger drug company.

The stock price topped $12 before that announcement.