A construction worker who drank four to five energy drinks every day for three weeks, was hospitalised with severe liver problems "likely" caused by excessively imbibing the seemingly harmless substance, researchers said.
The man, 50, was admitted to a hospital in Florida after feeling unwell for two weeks, with abdominal pain quickly progressing to nausea and vomiting.
He thought his symptoms were flu-like, but became alarmed when his urine became dark and his skin and the whites of his eyes turned yellow.
The man had recently started taking four to five daily servings of "a common energy drink" to help him get through long days at work -- the only change in his diet or habits, said a report in the journal BMJ Case Reports.
An examination revealed the man had severe hepatitis, a type of liver inflammation, and possible liver damage.
The symptoms were "likely" caused by excessive energy drink consumption, the team wrote.
It is the second known case of its kind, they added.
The man's daily intake of vitamin B3, or niacin, was about 160-200 milligrammes for the three-week period, "below the threshold expected to cause toxicity", the authors said, but similar to the previous known case.
"Toxicity is likely worsened by accumulative effect. Each bottle of his energy drink contained 40 mg of niacin or 200 percent of the recommended daily value," said a BMJ statement summarising the findings.
The man's symptoms disappeared on day three of hospitalisation and he was discharged on day six with instructions to avoid any niacin-containing products.
The observed link may have been a mere coincidence, and does not constitute proof that the drink caused the man's illness, the team underlined.
But it could be a red flag, they said.
"Based on this case and the previous report, we suggest that patients with pre-existing hepatic disorders should use caution when consuming energy drinks containing niacin," said the study.
Doctors should also be aware of the potential "adverse effects" and consider excessive energy drink consumption as a diagnostic option when otherwise healthy adults suddenly develop acute hepatitis.
The patient had previously contracted the Hepatitis C virus, but the team dismissed this as the cause of his illness, saying it had been too long ago.