What is small intestinal bacterial overgrowth (SIBO), and how do you get tested in the UK?

The short answer

SIBO (small intestinal bacterial overgrowth) means too many bacteria are living in the small intestine, where there should be relatively few. They ferment food early, which can cause bloating soon after eating, wind, pain and changes in bowel habit. In the UK, SIBO is usually tested privately with a home breath test, because it isn’t routinely offered on the NHS.

Bloating that starts within an hour of eating, a stomach that looks “six months pregnant” by the evening, and a diagnosis of IBS that never quite explained why: this is one of the most common stories I hear in clinic. For some of these people, small intestinal bacterial overgrowth (SIBO) is part of the answer.

What is small intestinal bacterial overgrowth?

Most of your gut bacteria live in the large intestine (the colon). The small intestine, where most food is digested and absorbed, normally has far fewer. In SIBO, bacteria build up in the small intestine. There they ferment carbohydrates from your food before you have had the chance to absorb them, releasing gas and sometimes competing with you for nutrients (Healthpath).

The body has several defences that normally keep numbers down: stomach acid, the regular “sweeping” movement of the small intestine between meals (the migrating motor complex), the valve between the small and large intestine, and a healthy immune lining. When one or more of these is weakened, SIBO becomes more likely.

What are the symptoms of SIBO?

SIBO symptoms are real but not specific, which is why testing matters. Common ones include:

bloating, often starting soon after meals
excess wind or belching
abdominal pain or discomfort
diarrhoea, constipation, or both
nausea
tiredness and brain fog
pale, greasy or floating stools in more severe cases

Because these overlap so much with IBS, food intolerances and other conditions, I never diagnose SIBO on symptoms alone.

What causes SIBO?

There is rarely a single cause. Things I look for include:

a past bout of food poisoning or gastroenteritis
slow gut movement, for example with an underactive thyroid, diabetes or some medicines
long-term use of acid-lowering medicines
previous abdominal surgery or adhesions
chronic stress, which affects gut movement and digestion
coeliac disease or inflammatory bowel disease

Finding the reason the bacteria built up is just as important as treating them. If you skip this step, SIBO often comes back.

How do you get tested for SIBO in the UK?

The usual test is a breath test, which you do at home. It’s specialised and not readily available on the NHS, so most people arrange it privately (Healthpath).

Here is how it works:

You follow a simple low-fermentation diet the day before.
You fast for 12 hours.
You give a baseline breath sample, then drink a sugar solution.
You collect breath samples at set times over about 2.5 hours.
The tubes go to a lab, which measures hydrogen and methane gases.

If bacteria are fermenting the sugar in your small intestine, the gases rise earlier than they should.

Glucose or lactulose: which sugar is better?

	Glucose breath test	Lactulose breath test
Where it reaches	Absorbed in the upper small intestine	Travels the whole small intestine
Main strength	Fewer false positives	Can pick up overgrowth further down
Main limitation	May miss overgrowth lower down	Fast transit, e.g. with diarrhoea, can give false positives
Suitable if lactose intolerant?	Yes	Usually yes, as lactulose is not lactose

Source: Healthpath.

I choose the test based on your symptoms and history, and I always interpret the result alongside them, never in isolation.

Why does methane matter?

Some people produce mainly methane rather than hydrogen. Methane tends to slow the gut down, so it is more often linked with constipation and bloating. It also changes how I approach treatment, so a test that measures both gases is worth having.

What does SIBO treatment involve?

Treatment usually happens in stages:

Reduce the overgrowth. Depending on the case, this may be with herbal antimicrobials or, through your GP or a gastroenterologist, a prescribed antibiotic.
Calm the symptoms. A short-term change in diet can help, but I avoid long, very restrictive diets, as they can harm the wider microbiome and your relationship with food.
Fix the cause. This means supporting digestion and gut movement, looking at stress and sleep, and reviewing medicines with your prescriber.
Rebuild. We gradually widen the diet and support a healthy microbiome.

You’ll find more on how I approach bloating and IBS in my gut health and IBS guide.

When should you see your GP first?

Please speak to your GP before any private testing if you have:

blood in your stools or black, tarry stools
unexplained weight loss
a lasting change in bowel habit, especially if you’re over 50
a lump in your tummy, or pain that wakes you at night
a family history of bowel cancer or inflammatory bowel disease

These need NHS assessment first (NHS). If you have severe abdominal pain, call 111 or, in an emergency, 999.

Common questions

Can SIBO be diagnosed on the NHS?

Breath testing for small intestinal bacterial overgrowth (SIBO) is not routinely available on the NHS, so most people in the UK use a private home breath test. Your GP can still rule out other causes of your symptoms, such as coeliac disease or inflammation.

Is SIBO the same as IBS?

No. Irritable bowel syndrome (IBS) is a diagnosis based on a pattern of symptoms. Small intestinal bacterial overgrowth (SIBO) is one possible cause of IBS-type symptoms that can be tested for. Some people with an IBS diagnosis have SIBO, and many don’t.

How long does SIBO treatment take?

Clearing small intestinal bacterial overgrowth the overgrowth often takes four to eight weeks. Fixing the underlying cause and rebuilding the gut can take a few months. Relapse is common when the cause isn’t addressed.

Can I do a SIBO test if I’m lactose intolerant?

Usually, yes. A small intestinal bacterial overgrowth breath test usually uses lactulose or glucose. Lactulose is a different sugar from lactose. If you have a true milk allergy, a glucose breath test may be a better choice.

If bloating is running your life, book a free discovery call and we can talk about whether SIBO testing makes sense for you.