The Food and Drug Administration said on Monday that it was investigating reports of five deaths that may be associated with Monster Beverage Corp's energy drink, and the company's shares fell more than 14 percent.
Monster is also being sued by the family of a 14-year-old girl who died after drinking two cans of its Monster Energy drink in a 24-hour period.
Monster said it does not believe its drinks are "in any way responsible" for the girl's death.
"Monster is unaware of any fatality anywhere that has been caused by its drinks," the company said in a statement. It said it intends to vigorously defend itself against the lawsuit.
The family of Anais Fournier filed a lawsuit on Friday against Monster for failing to warn about the product's dangers.
The lawsuit, filed in California Superior Court in Riverside, said that after drinking two 24-ounce cans of Monster Energy on consecutive days Fournier went into cardiac arrest. She died days later on December 23, 2011 from what the lawsuit said was "cardiac arrhythmia due to caffeine toxicity" that complicated a heart disorder she already had.
On Monday, FDA spokeswoman Shelly Burgess said the agency had received reports of five deaths and one heart attack that may be associated with the Monster Energy drink from 2009 through June this year.
The FDA said that it investigates any report of injury or death that it receives. The notices to the FDA's adverse events database do not in themselves confirm a risk from a product.
Monster is the leading U.S. energy drink by volume with nearly 39 percent of the U.S. market, but Austria's Red Bull has the highest share by revenue due to its premium price.
In July, New York State's Attorney General issued subpoenas to three energy drink makers -- Monster, PepsiCo and Living Essentials LLC -- seeking information on the companies' marketing and advertising practices.
PepsiCo makes the AMP energy drink and Living Essentials makes 5-Hour Energy.
Monster shares closed down 14.23 percent at $45.73 on the Nasdaq.
The case is Crossland et al v. Monster Beverage Corp, California Superior Court, Riverside County, No. RIC1215551.
(Reporting By Martinne Geller in New York; Editing by Gerald E. McCormick)