Irritable bowel syndrome: diagnosis and treatments that work

Key points

Irritable bowel syndrome (IBS) is common: it affects around 4 in every 100 adults under current criteria.⁴
It is diagnosed from the symptoms, using well-defined criteria and a few targeted tests. Chaining investigations together does not improve the outcome.^1,5
What changes treatment most is the subtype: constipation, diarrhoea or mixed.^2,3
What works: soluble fibre, a properly run low FODMAP diet, medicines aimed at the dominant symptom and low-dose amitriptyline.^6,7,8
Gut-brain therapies and exercise have trials behind them; IgG food intolerance tests do not.^3,9,10
Blood in the stool, weight loss, anaemia or new symptoms after the age of 50 do not fit IBS and need further investigation.

This article summarises the evidence and clinical guidelines so you can understand your situation. It does not replace an individual assessment.

Irritable bowel syndrome is one of the commonest reasons people see a gastroenterologist, and one of the most poorly resolved. Many people spend years with pain and bloating, go through several normal tests and come away feeling that there is “nothing wrong”, or that it is all down to stress. Neither is true.

In this article I explain how the diagnosis is made properly, which tests help and which are unnecessary, and which treatments have been shown to work in clinical trials.

What it is and what it is not

Irritable bowel syndrome is a disorder of gut-brain interaction. The bowel becomes more sensitive than normal and moves in an uncoordinated way. The result is abdominal pain related to opening your bowels, with changes in stool frequency or consistency, often together with bloating.¹

It is not an inflammatory disease, it does not damage the bowel and it does not increase the risk of bowel cancer. Nor is it “psychological”: stress can make it worse, just as it can make migraine worse, but the mechanisms are physical and measurable.²

~4 in 100adults meet the current criteria for IBS⁴

6 monthsof symptoms, at least, before calling it IBS¹

4 subtypesdepending on whether constipation, diarrhoea, both or neither predominate¹

How it is diagnosed

The diagnosis is made from the symptoms, using what are known as the Rome IV criteria. We call it IBS when there has been abdominal pain on at least one day a week over the last three months, starting at least six months ago, and the pain meets two or more of these conditions:¹

It is related to opening your bowels (it appears, changes or eases when you go).
It comes with a change in how often you open your bowels.
It comes with a change in the form or consistency of the stool.

This is what is called a positive diagnosis: it is recognised by its pattern, rather than reached after everything else has been ruled out. A clinical trial compared the two approaches, and the positive diagnosis, with a few basic tests, was just as safe and cheaper than testing for everything.⁵ Current guidelines recommend this approach.^2,3

“All my tests come back normal”

In IBS that is exactly what we expect. A normal colonoscopy or blood test does not mean nothing is wrong: it means the problem lies in how the bowel works, not in its structure. What helps is giving it a name and starting treatment.

Which tests you need

Fewer tests does not mean no tests. Before making the diagnosis, the conditions that resemble IBS need to be ruled out in a targeted way. For most people this is enough:^2,3

Test	What it is for
Basic blood test (full blood count and C-reactive protein)	Looking for anaemia or inflammation, which do not fit IBS²
Coeliac blood test	Coeliac disease is mistaken for IBS for years, especially with diarrhoea or bloating^2,3
Faecal calprotectin	A stool sample that distinguishes IBS from inflammatory bowel disease when there is diarrhoea^2,3
Colonoscopy	Not routinely. Yes if there are warning signs, symptoms starting after the age of 50 or a relevant family history³
Other tests	Only if the pattern suggests them: tests for bile acid malabsorption or microscopic colitis in persistent diarrhoea, or a lactose test if the link with milk is clear

What adds nothing are the IgG food intolerance tests advertised in pharmacies and clinics: they show that you have eaten a food, not that it disagrees with you, and lead to unnecessary restrictive diets.³ I have written in detail about breath tests for SIBO in Abdominal bloating and SIBO: which tests help.

Why the subtype matters

The subtype is defined by what your stools are like on days with symptoms, and it is what changes treatment most. A medicine that works very well in one subtype can clearly worsen another.^1,2

Subtype	What it looks like	Where treatment starts
With constipation	Hard or lumpy stools on most days with symptoms	Soluble fibre, osmotic laxatives and, if that is not enough, medicines that increase bowel secretion such as linaclotide^2,3
With diarrhoea	Loose or watery stools, often with urgency	Loperamide as needed for urgency, the low FODMAP diet and low-dose amitriptyline^2,8
Mixed	Alternating days of constipation and diarrhoea	Treating whichever symptom weighs most in each phase, taking care not to swing from one extreme to the other
Unclassified	No clear pattern predominates	The dominant symptom is treated, usually pain or bloating

Which treatments work

There is no pill that cures it, but there is a set of treatments with good evidence. They are usually combined and reviewed after a few weeks to see what works in your case.

Treatment	What we know
Regular exercise	In a clinical trial, increasing physical activity improved symptoms compared with keeping to the usual routine¹⁰
Soluble fibre (ispaghula or psyllium)	Improves overall symptoms. Insoluble fibre, such as bran, can worsen bloating⁶
Low FODMAP diet	Reduces symptoms in controlled trials, but must be done in phases and with reintroduction⁷
Antispasmodics and peppermint oil	Useful for cramping pain, particularly taken as needed^2,3
Low-dose amitriptyline	The ATLANTIS trial showed it improves symptoms when first-line measures are not enough. It acts on the sensitivity of the bowel, not because the problem is psychological⁸
Medicines by subtype	Linaclotide in the constipation subtype; loperamide for urgency in the diarrhoea subtype^2,3
Gut-brain therapies	Cognitive behavioural therapy and gut-directed hypnotherapy have trials behind them, especially when other treatments have not worked⁹
Probiotics	The evidence is weak. If tried, for around 12 weeks, and stopped if there is no improvement^2,3

About medicines “for nerves”

People often leave a consultation upset because they have been prescribed an antidepressant. It is worth explaining properly: in IBS these medicines are used at doses far below those used for depression, and what they do is turn down the volume of the pain signals the bowel sends to the brain. They are one of the most effective tools we have.

The low FODMAP diet, done properly

FODMAPs are short-chain sugars that are poorly absorbed and ferment in the bowel: they are found in wheat, onion, garlic, pulses, some fruits and milk. Reducing them improves symptoms in a good proportion of patients.⁷

The problem is how it is done. The diet has three phases: a few weeks of restriction, an orderly reintroduction to find out which foods genuinely cause problems, and a maintenance phase with the broadest diet possible. Staying indefinitely in the first phase, unsupervised, impoverishes the diet and alters the gut flora. That is why guidelines recommend doing it with the support of a dietitian or specialist.²

Common myths

“It’s just nerves.”

Stress can make it worse, but the mechanisms are physical: a more sensitive bowel, altered movement and changes in the gut flora.²

“If all my tests are normal, there’s nothing wrong.”

In IBS the tests are normal by definition. What fails is function, not structure.

“I need a colonoscopy to be sure.”

Only if there are warning signs, symptoms starting after 50 or a relevant family history.³

“An intolerance test will tell me what not to eat.”

IgG tests have no validity and lead to unnecessary diets.³

“I should cut out gluten and dairy for good.”

Coeliac disease must be ruled out first, while you are still eating gluten. Any restrictions that are needed are adjusted through an orderly reintroduction.

“There’s no treatment, you just have to live with it.”

Several treatments have trials behind them, and most people improve a great deal when the plan is matched to their subtype.^2,3

Warning signs: do not wait

See a doctor promptly if…

These symptoms are not typical of IBS and need fuller investigation, even if you have already been given that diagnosis:

Blood in the stool, or black stools
Weight loss without trying
Anaemia or iron deficiency
Symptoms that start or clearly change after the age of 50
Diarrhoea or pain that wakes you at night
Fever, or a family history of bowel cancer, coeliac disease or inflammatory bowel disease

How I approach it in clinic

I start by listening carefully to what your symptoms are like, which tests you have already had and what medication you take, because some commonly used medicines perpetuate the problem without anyone making the connection. That is usually enough to know whether it is IBS and which subtype.

Then I request only what is missing to rule out what matters, and I do not repeat what has already been done well. I explain what is happening in your bowel and why, and you leave with a written plan: what to change first, which treatment to start and when we will review whether it is working. No endless lists of forbidden foods and no supplements without evidence.

Living with IBS and not getting better?

We review your case, request only the tests that are missing and leave with a written treatment plan.

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Frequently asked questions

Can IBS turn into cancer?

No. IBS does not damage the bowel or increase the risk of bowel cancer. What matters is not to put down to IBS symptoms that do not belong to it, such as blood in the stool or weight loss.

Do I need a colonoscopy?

Not in most cases. It is indicated if there are warning signs, if symptoms start after the age of 50 or if there is a relevant family history. Without those factors, a blood test, a coeliac blood test and faecal calprotectin are more informative.³

Is it psychological?

No. Stress can trigger or worsen symptoms, but the mechanisms are physical: a more sensitive bowel, altered movement and changes in the gut flora.²

Should I stop eating gluten?

Not before coeliac disease has been ruled out, and the test is only reliable while you are eating gluten. Once coeliac disease is excluded, some people improve by cutting down on wheat because of its FODMAP content, not because of the gluten itself.²

How long does treatment take to work?

It depends on the treatment, but it is generally assessed after a few weeks. Amitriptyline is started at a very low dose and adjusted gradually. Probiotics, if tried, are continued for around 12 weeks.^2,8

Can it be cured?

It is a chronic condition that comes and goes, but it can be well controlled. The realistic and achievable goal is to go from organising your life around your bowel to having mild, occasional symptoms.

References

Lacy BE, Mearin F, Chang L, et al. Bowel disorders. Gastroenterology. 2016. PubMed
Vasant DH, et al. British Society of Gastroenterology guidelines on the management of irritable bowel syndrome. Gut. 2021. PubMed
Lacy BE, et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. Am J Gastroenterol. 2021. PubMed
Sperber AD, et al. Worldwide prevalence and burden of functional gastrointestinal disorders, results of Rome Foundation Global Study. Gastroenterology. 2021. PubMed
Begtrup LM, et al. A positive diagnostic strategy is noninferior to a strategy of exclusion for patients with irritable bowel syndrome. Clin Gastroenterol Hepatol. 2013. PubMed
Moayyedi P, et al. The effect of fiber supplementation on irritable bowel syndrome: a systematic review and meta-analysis. Am J Gastroenterol. 2014. PubMed
Halmos EP, et al. A diet low in FODMAPs reduces symptoms of irritable bowel syndrome. Gastroenterology. 2014. PubMed
Ford AC, et al. Amitriptyline at low-dose and titrated for irritable bowel syndrome as second-line treatment in primary care (ATLANTIS): a randomised, double-blind, placebo-controlled, phase 3 trial. Lancet. 2023. PubMed
Black CJ, et al. Efficacy of psychological therapies for irritable bowel syndrome: systematic review and network meta-analysis. Gut. 2020. PubMed
Johannesson E, et al. Physical activity improves symptoms in irritable bowel syndrome: a randomized controlled trial. Am J Gastroenterol. 2011. PubMed

Links go to PubMed or the original publication and open in a new tab.

Irritable bowel syndrome: how it is diagnosed and which treatments work