According to a study conducted by Fukuoka University, the man in his 20s likely died of consuming too many caffeinated drinks, Xinhua reported.
The death was treated as accidental, despite traces of a caffeine pill being found in his stomach, as there were no indications of foul play or a deliberate overdose, the study said.
Japan's Health, Labour and Welfare Ministry said it was the first such case that had been reported and stated: "We had never heard of fatal caffeine intoxication."
The deceased apparently routinely drank highly-caffeinated drinks to stay awake during his job at a 24-hour gasoline stand.
One company that manufactures such "energy drinks" warns against drinking numerous cans or bottles of its product and for its consumers to not mix the drink with alcohol as it may cause side effects.
In terms of caffeine consumption, Japan's Food Safety Commission, states somewhat ambiguously that three cups of coffee a day for a healthy adult and a 350-mm can of cola for children between four and six years old is an "adequate" amount.
While previously unreported in Japan, deaths from caffeine overdose are common in the US where highly-caffeinated drinks are widely available and commonly served alongside alcohol in bars.
Such drinks are also available in urban bars and clubs in Japan with vodka often being added to the drink.
The drinks themselves are available at almost all convenience stores and supermarkets throughout the country, with the companies' TV commercials often using overworked salary men or athletes to promote their "energy giving" products.
According to police reports, the deceased had returned home and slept. At some point thereafter he vomited and was rushed to the hospital where he died.
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