[T]he simplest explanation was that he was drinking when nobody was looking. So doctors put him in an isolated room for 24 hours, watching his blood alcohol level. Sure enough, without a drink, the alcohol level in his blood rose 0.12 percent. Turns out the man’s own stomach, colonized by brewer’s yeast, was brewing beer—a condition doctors call “auto-brewery syndrome.” The doctors described the case in the International Journal of Clinical Medicine:

Gut Fermentation Syndrome also known as Auto-Brewery Syndrome is a relatively unknown phenomenon in modern medicine. Very few articles have been written on the syndrome and most of them are anecdotal. This article presents a case study of a 61 years old male with a well documented case of Gut Fermentation Syndrome verified with glucose and carbohydrate challenges. Stool cultures demonstrated the causative organism as Saccharomyces cerevisiae. The patient was treated with antifungals and a low carbohydrate diet and the syndrome resolved. Helicobacter pylori was also found and could have been a possible confounding variable although the symptoms resolved post-treatment of the S. cerevisiae.