IBS-D, IBS-C & IBS-M: Which Type of IBS Do You Have?
The three IBS subtypes explained — how they are defined, how to work out which is yours, and why the answer changes the treatment.
Part of the Complete IBS Guide.
Key fact: IBS subtypes are decided by stool form, not frequency, and only the abnormal days count. That distinction matters because the treatments diverge sharply — loperamide settles IBS-D and worsens IBS-C, while a bulk-forming laxative does the reverse.
Why the subtype matters
IBS is one diagnosis with three quite different day-to-day realities. Someone with IBS-D plans their life around toilet access; someone with IBS-C spends days uncomfortable and bloated between bowel movements; someone with IBS-M gets both, often in the same week. The symptom that dominates decides almost everything about treatment — which medicine helps, which laxative to avoid, which fibre to add, and which dietary change is worth the effort.
Getting the subtype right is also how you avoid making things worse. A bulk-forming laxative that helps IBS-C can worsen bloating in IBS-D. Loperamide, which settles IBS-D within hours, will make IBS-C considerably more uncomfortable. The subtype is not a label — it is the instruction manual.
How the subtypes are defined
Subtyping is done on stool form, not frequency, using the Bristol Stool Form Scale, and only counting the days when your bowels are abnormal. The scale runs from type 1 (separate hard lumps) to type 7 (entirely liquid), with types 3 and 4 being normal.
| Subtype | Definition (of abnormal stools) | What it feels like |
|---|---|---|
| IBS-C (constipation) | More than a quarter hard or lumpy (types 1–2), fewer than a quarter loose | Straining, infrequent or incomplete emptying, bloating that builds over days |
| IBS-D (diarrhoea) | More than a quarter loose or watery (types 6–7), fewer than a quarter hard | Urgency, morning clustering of stools, fear of being caught out |
| IBS-M (mixed) | More than a quarter hard and more than a quarter loose | Swinging between the two, often within the same week |
| IBS-U (unclassified) | Meets IBS criteria but the stool pattern fits none of the above | Pain and bloating dominate; bowel habit is close to normal |
How to work yours out: record stool form on the Bristol scale for two weeks, along with pain and bloating. Two weeks of honest recording will place you far more accurately than trying to remember a typical month — and it is the single most useful thing to bring to an appointment.
Underneath all three sits the diagnosis itself. NICE defines IBS as abdominal pain or discomfort that is either relieved by opening your bowels or associated with a change in stool frequency or form, plus at least two of: altered stool passage such as straining or urgency, bloating or visible distension, symptoms made worse by eating, and the passage of mucus. Our IBS condition page covers the full diagnostic picture.
IBS-D: diarrhoea-predominant
The defining features are urgency and unpredictability rather than volume. Stools often cluster in the morning or after meals, driven by an exaggerated gastrocolic reflex — the normal signal that tells the colon to move when the stomach fills. Many people with IBS-D eat less during the day simply to avoid triggering it, which tends to make everything worse.
What tends to help: loperamide is the first-line antimotility treatment and is more useful taken pre-emptively before a trigger situation than as a rescue afterwards — start low, because too much swings you into constipation. Soluble fibre such as ispaghula can firm stools; insoluble fibre such as bran usually makes things worse. Sorbitol and other sugar alcohols in sugar-free gum and drinks are a common hidden trigger. Caffeine within three cups a day is the NHS ceiling, and alcohol and fatty meals are frequent culprits.
Before settling on IBS-D: coeliac disease, bile acid diarrhoea and inflammatory bowel disease all present this way. NICE recommends coeliac serology for anyone with these symptoms, and a faecal calprotectin test helps separate inflammatory bowel disease from IBS. Diarrhoea that wakes you at night, or comes with blood or weight loss, is not IBS until proven otherwise.
IBS-C: constipation-predominant
IBS-C is not simply constipation. The distinguishing feature is pain that is tied to bowel habit, and a sense of incomplete emptying that persists even when stools are passed. Bloating is usually the worst symptom and builds across the days between bowel movements.
What tends to help: gradually increased soluble fibre — oats, ispaghula, linseed — plus fluid and regular movement. Where a laxative is needed, an osmotic such as macrogol is the usual choice. NICE specifically advises against lactulose in IBS, because it is fermented in the colon and generates exactly the gas you are trying to avoid. Stimulant laxatives are for short-term rescue rather than routine use. Where constipation has resisted several laxatives at maximum tolerated dose for at least 12 months, prescription options such as linaclotide exist and are worth discussing with a clinician.
IBS-M: mixed
IBS-M is the most frustrating subtype to treat, because the intervention that helps one half of the pattern aggravates the other. The approach that works is to stop treating the swings and start treating the baseline: regular meals at regular times, steady soluble fibre, adequate fluid, and treatment aimed at pain and bloating rather than at stool form.
Antispasmodics and peppermint oil earn their place here because they address the cramping and bloating that persist through both phases. Loperamide and laxatives are best kept as occasional, deliberate corrections rather than daily medicines — and it helps to accept that you are aiming for fewer extremes rather than a perfect bowel habit.
IBS-U and post-infectious IBS
IBS-U describes people who meet the criteria but whose stool form stays close to normal; pain and bloating dominate. If bloating is the symptom that bothers you most, our bloating page works through the wider differential.
Post-infectious IBS is worth knowing about because it has a clear story: symptoms that begin after an episode of gastroenteritis and persist long after the infection has cleared. It is usually diarrhoea-predominant, it is common after bacterial food poisoning, and it carries a better long-term outlook than IBS that has been present for years — a substantial proportion of people improve over the following months.
When it isn’t IBS
IBS is a positive diagnosis, but it is made only once the features that do not belong to it have been excluded. See a GP rather than self-managing if you have:
- Unexplained weight loss or loss of appetite
- Rectal bleeding, blood in the stool, or black tarry stools
- Diarrhoea that wakes you at night — IBS characteristically spares sleep
- A change in bowel habit lasting six weeks or more, particularly over 60
- Tiredness or breathlessness suggesting iron deficiency anaemia
- A family history of bowel or ovarian cancer, coeliac disease or inflammatory bowel disease
- New IBS-type symptoms over the age of 50 — NICE advises testing women in this group for ovarian cancer, because IBS rarely appears for the first time at that age
Treatment, by subtype
| First line | Medicines | Avoid | |
|---|---|---|---|
| IBS-D | Regular meals, reduce caffeine, alcohol, fat and sorbitol | Loperamide, antispasmodics, peppermint oil | Insoluble fibre (bran), sugar-free sweets |
| IBS-C | Soluble fibre built up slowly, fluid, activity | Macrogol, ispaghula, antispasmodics | Lactulose, routine stimulant laxatives |
| IBS-M | Regular meal timing, steady soluble fibre | Antispasmodics, peppermint oil; loperamide and laxatives as occasional corrections | Chasing each swing with a new medicine |
Across all three, the first-line medicines are antispasmodics — we compare mebeverine, Buscopan and alverine in detail, and cover what mebeverine is used for separately. Where diet is the main lever, the low FODMAP diet is the structured second-line approach, and it works best under a dietitian.
Frequently Asked Questions
How do I know which type of IBS I have?
Record your stool form on the Bristol Stool Form Scale for two weeks, counting only the days your bowels are abnormal. If more than a quarter of those stools are hard or lumpy and fewer than a quarter loose, that is IBS-C. If more than a quarter are loose or watery and fewer than a quarter hard, that is IBS-D. If more than a quarter fall into each category, that is IBS-M. Frequency matters far less than form.
Can your IBS subtype change over time?
Yes, and it commonly does. People move between subtypes over months and years, particularly between IBS-M and either of the other two, and treatment that stops working is often a sign the pattern has shifted rather than that the medicine has failed. It is worth re-recording your stool form for a fortnight whenever your symptoms change character.
Is IBS-D more serious than IBS-C?
Neither is more serious in the sense of causing long-term damage, because IBS does not injure the bowel. IBS-D does carry more diagnostic overlap with conditions that do matter — coeliac disease, bile acid diarrhoea and inflammatory bowel disease — so it usually warrants more testing before the diagnosis is settled.
Why does the same food affect me differently on different days?
IBS symptoms depend on the state of the gut when the food arrives, not on the food alone. Stress, sleep, the menstrual cycle, how fast you ate and what else was already in the gut all change the response. This is why single-food elimination often gives confusing results and why a structured approach such as the low FODMAP diet exists.
Does IBS ever go away?
It fluctuates rather than progresses. Many people have long quiet periods and flare-ups tied to stress, illness or dietary change. Post-infectious IBS, which starts after a bout of gastroenteritis, has the best outlook and a substantial proportion of people improve over the following months.
References
- National Institute for Health and Care Excellence. Irritable bowel syndrome in adults: diagnosis and management (CG61). nice.org.uk
- National Institute for Health and Care Excellence. Suspected cancer: recognition and referral (NG12). nice.org.uk
- NHS. Irritable bowel syndrome (IBS): diet, lifestyle and medicines. nhs.uk
- Monash University. Starting the low FODMAP diet. monashfodmap.com
- National Institute for Health and Care Excellence. Coeliac disease: recognition, assessment and management (NG20). nice.org.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. IBS should be diagnosed by a clinician who has excluded the conditions that mimic it. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.