Constipation means passing stools less often than is normal for you, or straining to pass stools that are hard, dry or lumpy. It is one of the most common digestive complaints in the UK, and in most cases the explanation is ordinary: not enough fibre or fluid, a change in routine, a new medicine, or regularly ignoring the urge to go. Most constipation improves with changes you can make yourself, and this guide covers what the evidence supports, where your gut bacteria fit in, and the small number of symptoms that mean you should see a GP rather than adjust your diet and wait.
What counts as constipation?
There is no single normal. Anything from three bowel movements a day to three a week sits within the range seen in healthy adults, so the useful comparison is with your own usual pattern rather than anyone else's. Doctors generally consider constipation likely when you are going fewer than three times a week, when stools are hard or lumpy, or when you regularly strain or feel you have not fully emptied.
Consistency tells you as much as frequency. Clinicians use the Bristol stool chart, which grades stools from type 1, separate hard lumps, to type 7, entirely liquid. Types 1 and 2 point towards constipation, and types 3 and 4 are the range most people would recognise as comfortable. Nothing about an occasional firm stool needs attention. A pattern of them, lasting weeks, is worth acting on.
What causes constipation?
Most cases trace to everyday habits. A diet low in fibre gives the colon little bulk to work with, and low fluid intake makes what is there harder and slower. Sitting still for most of the day, a change of routine such as travel or shift work, and repeatedly putting off going to the toilet all slow things further. The bowel responds to habit more than most people expect, and ignoring its signals trains it to stop sending them.
Medicines are the cause people most often miss. Opioid painkillers such as codeine, some antidepressants, some blood pressure medicines, and iron supplements are all well known for causing constipation. If your bowel habit changed around the time a medicine started, that timing is worth mentioning to a pharmacist or your GP. Never stop or reduce a prescribed medicine on your own account; there is usually a way to manage the side effect while staying on the treatment. Everyday painkillers affect the gut in other ways besides bowel habit, and our guide to how anti-inflammatory painkillers like ibuprofen and naproxen affect the gut covers the most widely used group.
Constipation is also common in pregnancy, driven by hormonal changes and, later, pressure from the growing baby. Anything you take or change during pregnancy, including fibre supplements and laxatives, should go through your midwife or GP first.
Sometimes constipation is one part of a wider pattern. If it alternates with diarrhoea, or comes with abdominal pain that eases after going to the toilet, and this has carried on for months, that pattern is worth reading about in our complete guide to IBS, because constipation-predominant IBS is a recognised form of the condition. A medical review classifies ongoing constipation without such a cause into slower-than-normal transit, normal transit with hard or difficult stools, or a difficulty with the emptying itself, which is one reason persistent cases need a GP conversation rather than indefinite self-management.
Are your gut bacteria involved?
They appear to be, though the direction of the relationship is still being worked out. A review from King's College London reported that adults with ongoing constipation tend to carry noticeably fewer Bifidobacterium and Lactobacillus species, the groups usually described as beneficial, than people with a comfortable bowel habit. People with constipation are also more likely to produce methane on breath tests, which matters because the gut microbes that make methane are associated with slower movement through the colon in several studies.
Most of this evidence shows association rather than cause. Slower transit changes the environment bacteria live in, so an altered microbiome may be a consequence of constipation as much as a contributor to it, and researchers openly disagree about how much each direction explains. What the association does tell you is that the things that feed a broader mix of gut bacteria, chiefly a varied, fibre-rich diet, are the same things that improve constipation, so the practical advice points one way regardless. If bloating is part of your picture too, our guide to bloating after eating covers that side.
What helps, ranked by the evidence
Fibre comes first, and it is the best evidenced change you can make yourself. A 2022 King's College London analysis of 16 randomised trials in 1,251 adults with chronic constipation found that two thirds of people responded to added fibre against four in ten on placebo, with psyllium the best supported type, amounts above 10 grams a day working better than smaller ones, and at least four weeks needed to judge the effect. A separate four-week trial in 250 adults with constipation, part funded by a supplement manufacturer, found fibre formulas including psyllium softened hard stools where placebo did not. The main side effect in those trials was extra wind, especially at the start. Build up gradually rather than jumping to a high intake overnight, and give it a month before deciding it has failed. Fibre from food counts just as much as supplements, and variety helps the wider gut for reasons beyond constipation; our guide to the best foods for gut health covers what that looks like on a plate.
Kiwifruit has stronger trial evidence than any other single food. In an international trial of 184 adults, eating two green kiwifruits a day for four weeks increased complete bowel movements by about one and a half a week in people with constipation and improved comfort, with no meaningful side effects. Two caveats apply: the trial was part funded by a kiwifruit marketing company, and a separate analysis found kiwifruit extract capsules did nothing, so it is the whole fruit or not at all.
Fluid and movement help, more modestly. Increasing fluid mainly helps if you are drinking little, and gentle regular activity is associated with better bowel habit even though trial evidence is thin. Neither will fix constipation alone, and both are cheap to try. Routine matters too: going when the urge comes rather than deferring it, unhurried time on the toilet after breakfast when the bowel is naturally most active, and a footstool to raise your knees, which straightens the angle of the rectum and makes emptying easier.
What about supplements and laxatives?
Live culture supplements have been tested in constipation, and the results are mixed. A meta-analysis of 14 randomised trials in 1,182 adults found that, on average, they modestly increased stool frequency and softened consistency, with Bifidobacterium lactis strains accounting for most of the benefit and other strains showing none. The authors flagged high variability and bias risk across the trials, so this is a may-help, not a will-help. If you try one, the strain matters more than the brand.
Magnesium-based laxatives have surprisingly strong trial support, and pharmacy shelves hold several other types that work in different ways. Which one suits you depends on your situation and what else you take, which is exactly what a community pharmacist is for; you do not need an appointment. Two rules hold regardless. Laxatives are a short-term measure while diet and routine changes take effect, not a permanent answer, and constipation that keeps returning whenever you stop them is a reason to see your GP, not to keep buying them.
Can a gut test tell you anything useful?
Constipation is a symptom you can observe directly, so no test is needed to know you have it. Where testing offers something is the layer you cannot observe: which bacteria your gut is carrying. Given the research above linking constipation with lower levels of specific groups such as Bifidobacterium, a gut microbiome test for constipation shows you where your own levels sit and gives you a concrete before-and-after if you overhaul your fibre intake. It does not diagnose the cause of constipation, it cannot rule anything out, and it is not a substitute for the GP conversation below if any red-flag symptom is present. A snapshot of your gut bacteria is context for the changes you make, not a verdict.
When should you see a GP?
Most constipation does not need a doctor. The following do. Book a GP appointment if you have:
- A change in your bowel habit lasting more than three weeks, especially if you are over 50
- Blood in your stool, or bleeding from the bottom
- Unexplained weight loss
- Constant abdominal pain, or a lump in your abdomen
- Constipation alternating with diarrhoea over a sustained period
- Unexplained tiredness, which can accompany iron-deficiency anaemia
- A family history of bowel or ovarian cancer alongside a persistent change
Seek same-day advice if you have severe abdominal pain with vomiting and no wind or stool at all, because a blocked bowel is an emergency. In children, constipation is managed differently and is a matter for a GP or health visitor, not home experimentation. None of the symptoms above is investigated at home, and no home test replaces that assessment.
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
- Vriesman MH, et al. (2019). Management of functional constipation in children and adults. Nature Reviews Gastroenterology & Hepatology. PubMed | DOI
- van der Schoot A, et al. (2022). The effect of fiber supplementation on chronic constipation in adults: an updated systematic review and meta-analysis of randomized controlled trials. The American Journal of Clinical Nutrition. PubMed | DOI
- Gearry R, et al. (2023). Consumption of 2 green kiwifruits daily improves constipation and abdominal comfort: results of an international multicenter randomized controlled trial. The American Journal of Gastroenterology. PubMed | DOI
- van der Schoot A, et al. (2023). The effect of food, vitamin, or mineral supplements on chronic constipation in adults: a systematic review and meta-analysis of randomized controlled trials. Neurogastroenterology & Motility. PubMed | DOI
- Dimidi E, et al. (2014). The effect of probiotics on functional constipation in adults: a systematic review and meta-analysis of randomized controlled trials. The American Journal of Clinical Nutrition. PubMed | DOI
- Dimidi E, et al. (2017). Mechanisms of action of probiotics and the gastrointestinal microbiota on gut motility and constipation. Advances in Nutrition. PubMed | DOI
- Lai H, et al. (2023). Effects of dietary fibers or probiotics on functional constipation symptoms and roles of gut microbiota: a double-blinded randomized placebo trial. Gut Microbes. PubMed | DOI
- NHS. Constipation. nhs.uk
