The US Food and Drug Administration is investigating five deaths and a heart attack for possible links to consuming Monster energy drinks, an agency spokeswoman said.
"I can verify that FDA has received five adverse event reports of death and one of heart attack possibly associated with Monster energy drink," the FDA spokeswoman, Shelly Burgess, said.
Burgess, however, said such reports "serve as a signal to FDA and do not prove causation between a product or ingredient and an adverse event".
She said such reports were taken seriously and diligently investigated by the agency.
Burgess urged consumers who have experienced an adverse reaction to an energy drink to notify the manufacturers, which are required to report them to the FDA within 15 days.
The family of teenager Anais Fournier, from the US state of Maryland, who died of an abnormal heartbeat in December, allegedly after drinking two cans of Monster over a 24 hour period, filed a lawsuit last week in California against Monster Beverage.
Her parents accused the company of not warning consumers of the potential dangers.
"I was shocked to learn the FDA can regulate caffeine in a can of soda, but not these huge energy drinks," said Wendy Crossland, the girl's mother, who called the beverages "death traps" for adolescents.
According to the complaint, the two cans of energy drink that Fournier consumed contained about 480 milligrams of caffeine - equal to 14 Coca-Colas.
The drink's manufacturer, Monster Beverage Corporation, denied any responsibility for Fournier's death.
"Monster does not believe that its products are in any way responsible for the death of Ms Fournier and intends to vigorously defend the lawsuit," it said in a statement.
"Neither the science nor the facts support the allegations that have been made. Monster reiterates that its products are and have always been safe."
A November 2011 report by the US Department of Health and Human Services found a tenfold spike in emergency room visits for issues linked to energy drinks between 2004 and 2009, with more than 16,000 visits in 2008. Sales of the beverages skyrocketing during that period.
Information about the report was released in a statement from Kevin Goldberg, a lawyer representing Fournier's family.
The firm Beverage Digest estimates that the sales of energy drinks, representing 3 per cent of the soda market, rose by about a sixth last year.
Aggressive marketing by energy drink companies targeting youngsters has led to growing concern among authorities.
Energy drinks contain stimulants such as caffeine, guarana and ginseng, the relaxant taurine and other vitamins.