A man developed an acute liver problem because of his daily energy drink intake, doctors suggest.
The case occurred in Florida when a 50-year-old construction worker sought help after developing vomiting, jaundice and abdominal pain.
He already had an underlying liver problem, which doctors think increased the risk from his four or five drinks per day.
British experts said it added up to a "double whammy" for his liver.
The patient originally thought he had flu, but sought medical advice when he developed other symptoms including dark-coloured urine and jaundice.
Writing in BMJ Case Reports, the medical team from the University of Florida College say the man did already have chronic hepatitis C - a liver condition - but these symptoms developed separately.
Tests showed he had high levels of liver enzymes, called transaminases, which are a sign of liver damage, as well as evidence of his chronic hepatitis C infection.
A biopsy revealed severe hepatitis.
To try to find out what had caused his condition, they asked the patient about any smoking, alcohol and illicit drug use and diet.
But the only thing which he reported as different from the norm was his use of energy drinks to get him through his working day - four to five drinks a day over the preceding three weeks.
Each had around 40mg of niacin (vitamin B3) - UK recommendations are that men have no more than 17mg a day and women 13mg.
People usually experience toxic symptoms only if they have more than 500mg, but in this man's case, 160mg-200mg a day was enough to affect his liver.
His symptoms disappeared when he stopped consuming energy drinks.
This is only the second such case to be reported in medical journals.
The researchers, led by Dr Jennifer Harb, said: "As the energy drink market continues to rapidly expand, consumers should be aware of the potential risks of their various ingredients.
"Vitamins and nutrients, such as niacin, are present in quantities that greatly exceed the recommended daily intake, lending to their high risk for harmful accumulation and toxicity."
'Your body's factory'
Her colleague Dr Vikas Khullar told the BBC that everyone, not just those with hepatitis or other liver disorders, should be cautious about how many energy drinks or herbal teas they drink.
He said: "We are not sure how many of the ingredients are filtered through the liver, and what levels may lead to toxicity and liver injury."
Andrew Langford, chief executive of the British Liver Trust, said: "The liver is your body's factory - it carries out hundreds of functions that are vital to life.
"These functions include destroying and dealing with drugs or toxins, processing food and drink once it has been digested and storing energy so that it can be used effectively.
"Energy drinks offer no nutritional benefit and drinking too many of them can stop the liver from doing its job properly and lead to serious problems."
He added having a pre-existing liver condition was "likely to exacerbate the problem and create a 'double whammy' effect".
Mr Langford said a balanced diet - including plenty of water, exercising regularly and limiting alcohol consumption - would help maintain a healthy liver,