When IBS is diagnosed, it usually comes with one of three subtype labels. IBS-C means constipation is your dominant bowel pattern, IBS-D means diarrhoea dominates, and IBS-M means both happen regularly, sometimes within the same week. The label is worked out from the form of your stools on the days your bowel habit is abnormal, using a chart called the Bristol stool form scale, and it can change over time as your pattern changes. A subtype does not change what IBS is or how serious it is. It guides which treatments a GP or pharmacist considers first. This guide covers how each subtype is defined, what each tends to mean day to day, why the labels shift, and where treatment differs between them.
How are the IBS subtypes worked out?
Doctors use an international set of definitions called the Rome IV criteria, and the Rome IV description of bowel disorders bases the subtypes entirely on stool form. What counts is not how often you go, but what your stools are like on the days your bowel habit is abnormal.
Stool form is scored on the Bristol stool form scale, which runs from type 1 (hard, separate lumps) to type 7 (entirely liquid). The scale was developed in Bristol in the 1990s, and the study behind it, in 66 volunteers, showed that stool form tracks how quickly food is moving through the gut. Hard, lumpy stools mean a slow transit, loose and watery ones a fast transit, which is why the same chart can describe both ends of IBS.
- IBS-C. On the days your stools are abnormal, more than a quarter are hard or lumpy (Bristol types 1 and 2) and fewer than a quarter are loose or watery (types 6 and 7)
- IBS-D. The reverse. More than a quarter loose or watery, fewer than a quarter hard or lumpy
- IBS-M. More than a quarter hard or lumpy and more than a quarter loose or watery. The M stands for mixed
- IBS-U. A smaller fourth group, unclassified, where the abnormal stools do not meet either threshold
Because medicines for constipation or diarrhoea change stool form, the pattern is judged from what your bowels do without them, which is why a GP may ask what your stools were like before you started taking anything for them. Subtyping happens after IBS itself has been diagnosed, and how IBS is diagnosed in the UK is a separate step, made by a GP from your symptoms and history once other conditions have been ruled out.
What each subtype means day to day
The definitions are about stool form, but the subtypes feel different to live with, and research is starting to map those differences. In a 2026 analysis of 2,195 people with IBS across 26 countries, part of a worldwide survey run by the Rome Foundation, the pain profiles of the subtypes were clearly distinct.
IBS-C
Stools are hard and infrequent, often with straining and a feeling of not having finished. Bloating tends to build through the day. In the survey above, people with IBS-C reported pain that lasted longer, spread more widely and more often radiated to the back than in the other subtypes, and they were the group most dissatisfied with how their bowels worked.
IBS-D
Stools are loose or watery, often with urgency, and symptoms commonly arrive in the mornings or after meals. In the same survey, pain in IBS-D was tied closely to changes in how often people went and how loose their stools were, easing and worsening with the bowel pattern itself. Under the current criteria IBS-D is the most common subtype. A pooled analysis of surveys covering more than 82,000 adults found around 4% of people worldwide meet the strict Rome IV definition of IBS, that IBS-D accounts for the largest share of them at roughly a third, and that IBS overall is about one and a half times more common in women, a difference explored further in our guide to IBS in women.
IBS-M
Bowel habit swings between the two, hard stools for a stretch and then loose ones, sometimes within a single day. Many people with IBS-M describe the unpredictability as the hardest part, because planning around symptoms is harder when the symptoms will not pick a direction. It also complicates the medicine cabinet. Something that eases constipation can tip you towards diarrhoea and the other way round, which is why anything used for IBS-M tends to be short-term and reviewed rather than taken on a fixed schedule.
Can your subtype change over time?
Yes, and it happens often enough that researchers have measured it. In a Danish study that followed 126 people with IBS through ten weeks of daily stool diaries, only around a fifth to a third kept exactly the same subtype every single week. Underneath that week-to-week movement most people were more consistent than they looked. Over 80% had one pattern that dominated at least 60% of the time.
The practical reading is that a subtype describes your usual pattern, not a fixed identity. A label given at diagnosis years ago may no longer fit the gut you have now, and it is reasonable to say so at your next GP appointment, because the label steers which treatments are tried. A bad week on its own is different. Symptoms that surge and then settle back to your normal are the shape of an IBS flare-up rather than a change of subtype. What should never be put down to a shifting subtype is a change in bowel habit that persists for more than three weeks, which needs a GP appointment whichever label you carry.
How treatment differs by subtype
The starting point is the same for all three. NHS advice begins with regular meals, plenty of fluid, limits on caffeine, alcohol and fizzy drinks, and a diary to identify your own triggers, whichever subtype you have. Food is where most people look next, and the diets used to manage IBS each carry different evidence, with the more restrictive ones needing a dietitian's support.
The subtype starts to matter at the pharmacy and in the GP surgery, because most IBS medicines target one end of the bowel spectrum. The British Society of Gastroenterology's 2021 guideline and NHS advice line up on the broad shape.
- For IBS-C. Soluble fibre such as ispaghula husk is the usual first step, increased gradually, alongside enough fluid. A pharmacist can advise on suitable laxatives, and for constipation that has not responded over a long period a GP has further prescription options
- For IBS-D. The NHS suggests cutting down on insoluble fibre, found in wholegrains, nuts and seeds, and names loperamide as the first-choice pharmacy medicine for the diarrhoea itself. A pharmacist can check it suits you
- For IBS-M. Treatment follows whichever pattern is causing trouble at the time, which is where a stool diary earns its keep, and medicines are used more sparingly because of the tipping effect described above
Several options are not subtype-specific at all. Antispasmodics for cramping, peppermint oil, low-dose antidepressants prescribed for gut pain signalling, and psychological therapies directed at the gut are used across the subtypes, matched to symptoms rather than to the label. All of the prescription decisions belong to you and your GP, and none of these medicines should be started or stopped on your own. The full picture of how IBS is treated in the UK, including the stepped NHS pathway, has its own guide.
Can a gut microbiome test tell you your subtype?
No. Researchers have compared the gut bacteria of people with different IBS subtypes, and the picture is unsettled. A 2024 systematic review of studies using one standardised stool test found the results inconsistent between subtypes, with the most evidence of bacterial imbalance in IBS-D and no bacterial signature that separates one subtype from another. Differences in gut bacteria are a live research thread in IBS, not a classification tool. What microbiome testing for IBS can show is the balance of bacteria you were carrying when you sampled, which some people use as context for a GP conversation about their gut.
The limits are firm. No stool test can diagnose IBS, assign your subtype, or tell you whether your pattern is about to change. A result is a snapshot of your bacteria on one day, and it shifts with diet and time.
The tool that does settle your subtype is unglamorous. A few weeks of noting your stools against the Bristol chart, on paper or in an app, gives a GP exactly the information the definitions are built on, and it costs nothing.
When a change in bowel habit needs a GP
Bowel habit changes are the defining feature of IBS, and they are also how several conditions that need investigation first show themselves. Book a GP appointment promptly if you notice any of the following, whether or not you already have an IBS diagnosis and whatever your subtype.
- A change in bowel habit lasting more than three weeks. A persistent new swing towards harder or looser stools is not a subtype shift until a GP has checked it, particularly if you are over 50
- Blood in your stool, bleeding from your bottom, or stools that are black and tarry
- Unexplained weight loss
- Bowel symptoms starting for the first time over the age of 50
- Symptoms that wake you from sleep, including needing to open your bowels at night
- Signs of anaemia, such as unusual tiredness, breathlessness or paler skin than usual
- A lump or swelling in your tummy or back passage
- A family history of bowel or ovarian cancer alongside new gut symptoms
One more caution belongs here. The subtype definitions only apply once IBS has been diagnosed. If you have not seen a GP about your symptoms yet, matching yourself to IBS-C or IBS-D from a description is a step too early, because constipation and diarrhoea have many causes and the point of diagnosis is to rule out the ones that need different treatment.
This guide is for information only and does not replace medical advice. Speak to your GP or pharmacist about your own symptoms and treatment.
Sources
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- NHS. Irritable bowel syndrome (IBS). Diet, lifestyle and medicines. nhs.uk
