Imagine you turn up to the hospital looking for help because you feel dizzy and can't stop slurring your speech – only to be told that you're simply drunk. Yet, you haven't had a drop of alcohol.

This recently happened to a woman in Canada, who experienced seven episodes of a mysterious illness that caused her to feel drunk even though she hadn't consumed any alcohol. It took two years before she finally received a diagnosis: she had "auto-brewery syndrome".

Auto-brewery syndrome is a very rare and little understood condition where microbes in your gut start to make alcohol.

Symptoms of auto-brewery syndrome share many similarities with signs of alcohol abuse, such as drowsiness, altered mood and vomiting. This makes accessing the right medical help difficult.

Indeed, another person with auto-brewery syndrome in Belgium found himself on the receiving end of a drink-driving charge despite not drinking any alcohol. He was later acquitted of his charges after several doctors agreed he was suffering from auto-brewery syndrome.

The exact number of people who have auto-brewery syndrome isn't known and there are only a handful of cases in the medical literature.

It may be there are many more people out there with the condition than we know of – but receiving a diagnosis can be difficult, which may limit the total numbers reported.

From the evidence gathered so far, it appears people are not born with auto-brewery syndrome. Instead, the illness is triggered by a disruption in the gut microbiome.

Yeast and fermentation

To make alcoholic drinks, we use yeast (a type of fungus) to ferment the sugars in carbohydrates. This generates the alcohol in beer, wine and spirits.

It's thought that people with auto-brewery syndrome have an overgrowth of similar types of yeast in their intestines. This ferments the food they eat, especially carbs like bread and sugar, and turns it into alcohol.

The strongest evidence we have to support this hypothesis is that antifungal drugs appear to help people with auto-brewery syndrome. Antifungal drugs kill yeast growing in the gut. Several patients with the condition have reported these treatments helped make their symptoms go away.

In the Canadian patient, a low carbohydrate diet also helped to reduce the number of "episodes" of slurred speech and dizziness she experienced.

Limiting how much yeast is in the gut – as well as the food sources yeast uses to grow – may help to slow down internal alcohol production. But it's important to note that we don't yet have specific clinical proof of this working in patients, so it remains a theory.

Exactly which yeast is responsible for auto-brewery syndrome isn't yet known. This is because of how rare the condition is, which limits how much we can learn about it.

But recent research has investigated the types of yeast that grow in the human gut – and what health benefits these microbes might deliver. This work might give us some clues about what's happening in people with auto-brewery syndrome.

One study compared alcohol production from different yeast species found in the human intestine. The results showed that a yeast called Saccharomyces cerevisiae was able to make the most alcohol in the tested conditions.

Saccharomyces is better known as "brewer's yeast", and is the same species that is commonly used to make bread and beer.

The same study also found alcohol production from other species which belonged to a family of yeasts called Candida, which commonly cause yeast infections in humans.

In patients with auto-brewery syndrome, a "spike" in the growth of yeasts like Saccharomyces and Candida in the gut may cause each episode of their illness.

This could be due to the types of food eaten, such as a high carbohydrate diet – and may be alleviated by following a low-carb diet as seen with the Canadian patient.

Other possible triggers of auto-brewery syndrome could also include the use of antibiotics or gut surgery, as both of these have been linked with increased gut fungi growth.

While Saccharomyces yeast are commonly reported in human microbiome studies, whether this yeast is a true resident of the human gut or simply passing through after we've eaten food containing the yeast (such as bread) is not clear.

Candida species, on the other hand, are well accepted as a regular inhabitant of the human microbiome – at least in the west.

Although yeast in our gut are much smaller in number than bacteria, they can still have large effects on our health and become a potential source of infection. Overgrowth of Candida in the gut has been associated with the development of fungal sepsis in cancer patients.

Patients with severe COVID have also been shown to have increased growth of Candida in their gut. Early growth of Candida in the gut of newborn babies is also associated with developing asthma in later life.

These associations are caused by the Candida in the gut affecting the function of the immune system. This then determines the likelihood of how well we fare with other infections (such as COVID) or developing immune-driven diseases such as allergies.

While auto-brewery syndrome may be rare, what is clear is that our gut fungi have wide-ranging effects on our health and immune system. We're only just beginning to understand how these fascinating yeasts affect our health.

By studying them further, we might just learn more about intriguing conditions similar to auto-brewery syndrome.