It began as a simple thumb injury. Then, it spiralled into a dangerous syndrome where a man's gut essentially became a brewery fermenting its own endless alcohol supply - which is not as fun as it sounds.

In a recent case study, doctors recount the strange symptoms of auto-brewery syndrome (ABS): a rarely diagnosed medical condition where simply ingesting carbohydrates can be enough to make you wildly inebriated. Even worse, nobody believes you when you say you haven't been drinking.

At least, that was the case for the unfortunate 46-year-old patient in question, an otherwise healthy man who'd only ever been a light social drinker.

His troubles began in 2011, after he completed a course of antibiotics for a thumb injury. Within one week of finishing the meds, he reported experiencing uncharacteristic personality changes, including depression, 'brain fog', aggressive behaviour, and memory loss.

He was eventually referred to a psychiatrist and given antidepressants, but it was only when the man was pulled over by police one morning in an apparent case of drunk driving that the true nature of his illness started to reveal itself.

When pulled over, he refused to take a breathalyser test and was hospitalised, with tests showing he had a blood alcohol level of 200 mg/dL, equivalent to having drunk approximately 10 alcoholic drinks, and sufficient to induce confusion, disorientation, impaired balance, and slurred speech.

"The hospital personnel and police refused to believe him when he repeatedly denied alcohol ingestion," researchers from Richmond University Medical Centre note in their case report.

After being discharged from hospital, he sought treatment at a clinic in Ohio. In medical tests, most of his readings looked normal, but his stool sample showed the presence of Saccharomyces cerevisiae (also known as brewer's yeast) and a related fungus.

S. cerevisiae has a long history in beer brewing and winemaking (in addition to baking), as it helps ferment carbohydrates and produces alcohol.

While the patient was successfully treated at the clinic, his ABS diagnosis revealed itself in subsequent flare-ups, with the most serious incident involving a fall while inebriated that resulted in intracranial bleeding.

While recovering in hospital, his blood alcohol spiked as high as 400 mg/dL – twice the concentration recorded when he was pulled over – but still "medical staff refused to believe that he did not drink alcohol despite his persistent denials", the researchers write.

Fortunately for the patient, he eventually sought treatment conducted in collaboration with the Richmond University specialists, who used a combination of anti-fungal therapies and probiotics to treat the man's gut microflora.

With the exception of one relapse – brought upon by a pizza and soda binge, unbeknownst to his treating doctors – the patient's fungal growths look to have been successfully treated.

"Approximately 1.5 years later, he remains asymptomatic and has resumed his previous lifestyle, including eating a normal diet while still checking his breath alcohol levels sporadically," the researchers explain.

It's a happy ending for the patient, who looks to be finally free not only of his unasked-for drunkenness (and resultant health problems), but also of the cloud of disbelief it invited in those around him.

"For years, no one believed him," one of the medial team, Fahad Malik, now a chief medical resident at University of Alabama at Birmingham, told New Scientist.

"The police, doctors, nurses and even his family told him he wasn't telling the truth, that he must be a closet-drinker."

As for how the auto-brewing fungus colonised the man's insides, the researchers suspect they started to set up their boutique brewery after the man received his thumb injury in 2011.

"We believe that our patient's symptoms were triggered by exposure to antibiotics, which resulted in a change in his gastrointestinal microbiome allowing fungal overgrowth," the authors explain, noting that we are only starting to recognise the complexity of this rare and probably under-diagnosed condition.

The findings are reported in BMJ Open Gastroenterology.