Shares of Idenix Pharmaceuticals Inc. plunged Thursday after the company said the Food and Drug Administration is again halting trials of its experimental hepatitis C drug.

The news comes about two weeks after Bristol-Myers stopped a clinical trial of a hepatitis C treatment because of an unspecified "serious safety issue."

As a result, the FDA has placed a hold on human testing of IDX184, Idenix's most advanced drug. The company said there were concerns about heart-related side effects from the drugs. It said no such side effects have been specifically linked to IDX184, although some heart side effects have been seen in patients treated with IDX184 and standard drugs.

Idenix shares dropped $2.35, or 28.3 percent, to $5.96 in morning trading.

On Aug. 1, Bristol-Myers Squibb Co. said it had suspended trials of a drug designated BMS-986094. It said it was not clear that the side effects in the trial were linked to the drug itself. IDX184 and BMS-986094 are both nucleotide inhibitors, meaning they are designed to prevent the hepatitis C virus from making copies of itself. Bristol-Myers did not immediately respond to request for comment.

The FDA placed a clinical hold on trials of IDX184 in 2010 because of concerns about the effects of the drug on patients' livers. It removed the clinical hold in February.

The company recently completed a mid-stage clinical trial of IDX184 and is preparing to report results from that trial. No patients are currently being treated with the drug. At a conference on Wednesday, Idenix President and CEO Ronald Renaud said the company is "quite pleased with the safety profile that we've seen to-date" for IDX184.

Shares of Achillion Pharmaceuticals Inc., a company that is developing a competing hepatitis C therapy, rose 24 cents, or 4.1 percent, to $6.14.

Hepatitis C is a virus that can lead to life-threatening liver damage and is the main cause of liver transplants in the United States. The disease is spread through the blood, and that can happen through sharing intravenous drug needles or having sex with an infected person. There are around 3 million Americans with the disease, which can go undetected for many years until the liver is severely damaged. More people will be diagnosed as the baby boomer generation ages.