When Constipation Is Not Just Constipation
The features that mean something else needs excluding, the tests that find it, and the pattern that laxatives will never fix.
Part of the Complete Constipation Guide.
Key fact: A persistent change in bowel habit lasting six weeks or more, particularly over the age of 50, is a reason to be assessed — not a reason to try a stronger laxative. So is any rectal bleeding, unexplained weight loss, or constipation that does not respond to treatment.
The red flags
Most constipation is functional and responds to the usual measures. These features are the ones that change the question from "which laxative" to "what is causing this".
- A persistent change in bowel habit lasting six weeks or more, particularly aged 50 or over
- Rectal bleeding, especially without anal symptoms such as pain or itching
- Unexplained weight loss
- A lump felt in the abdomen or in the back passage
- Iron deficiency anaemia, or persistent unexplained tiredness
- Stools that have become noticeably narrower
- Symptoms that wake you at night
- Fever alongside the bowel symptoms
- Constipation that does not respond to appropriate treatment
- A family history of bowel cancer or inflammatory bowel disease
Same-day assessment for severe abdominal pain with vomiting and an inability to pass wind, or a distended, tense and tender abdomen — these can indicate obstruction.
Why six weeks matters
Six weeks is long enough to exclude the ordinary explanations — a holiday, a course of codeine, a fortnight of poor eating, a bout of illness — and short enough to act on. The combination of a sustained change in habit and older age is the pattern that referral guidance is built around, because it is the presentation bowel cancer most often produces.
This is not a reason to panic about a fortnight of constipation after a long flight. It is a reason not to spend four months escalating laxatives without anyone asking why.
The FIT test
The faecal immunochemical test looks for tiny amounts of blood in the stool that are not visible. It has become the main tool for deciding who needs urgent investigation in primary care.
- The referral threshold is 10 micrograms of haemoglobin per gram of faeces or above
- A result below the threshold does not close the question. Guidance is explicit that a low result must not delay referral where clinical concern persists — symptoms outrank the number
- It is a single stool sample collected at home, not a hospital procedure
- It is not the same as the bowel screening kit sent routinely by post, although the technology is similar
The blood tests worth having
| Test | Looking for |
|---|---|
| Thyroid function | An underactive thyroid slows the gut, and usually comes with fatigue, cold intolerance, dry skin and weight gain |
| Calcium | A raised calcium level causes constipation, thirst, tiredness and confusion |
| Full blood count | Iron deficiency anaemia, which alongside a change in bowel habit is a significant combination |
| Glucose or HbA1c | Diabetes, particularly where nerve involvement has affected gut motility |
| Kidney function and electrolytes | Relevant to both the cause and the choice of laxative |
Hypothyroidism and hypercalcaemia are the two that most often turn up as the real explanation for constipation that would not settle.
Neurological causes
- Parkinson's disease — constipation frequently precedes the movement symptoms by years
- Multiple sclerosis
- Spinal cord injury or compression, and spina bifida
- Stroke, particularly with reduced mobility afterwards
- Diabetic autonomic neuropathy
New constipation with leg weakness, numbness in the saddle area, difficulty passing urine or loss of bladder control needs emergency assessment. That combination can indicate spinal cord compression, which is time-critical.
Medicines
Worth reviewing whenever constipation is new or worsening, because the timing is often the clue:
| Group | Examples |
|---|---|
| Opioids | Codeine, co-codamol, dihydrocodeine, tramadol, morphine — see opioid-induced constipation |
| Iron supplements | Ferrous sulfate and similar |
| Anticholinergic and antimuscarinic medicines | Tricyclic antidepressants, some antipsychotics, bladder antimuscarinics, some antihistamines |
| Calcium channel blockers | Verapamil in particular |
| Calcium and aluminium salts | Including some antacids and supplements |
| 5HT₃ antagonists | Ondansetron and related anti-sickness medicines |
Bring a full list, including anything bought over the counter. Co-codamol and antacids are the two most often left off.
When it is an outlet problem
Some people strain hard despite perfectly soft stool. That pattern suggests the problem is not transit but evacuation — the pelvic floor and anal sphincter failing to relax and coordinate properly.
- Straining with soft stool is the giveaway
- A sense of blockage at the outlet, or needing to press to help stool out
- More laxative does not help, and may make things messier without making them easier
- Treatment is physiotherapy — pelvic floor retraining and biofeedback — rather than medication
It is under-recognised and worth raising specifically, because the default response to unresolved constipation is to escalate laxatives, which is exactly the wrong direction here.
When it is IBS rather than constipation
If constipation comes with abdominal pain that is relieved by opening your bowels, and the pattern has been going on for months, IBS with constipation is the more useful label. It changes the approach: fibre has to be handled differently, insoluble fibre often makes things worse, and antispasmodics have a role that they do not have in simple constipation.
Fibre and IBS covers the distinction, and IBS red flags covers what still needs excluding before settling on that label.
What assessment involves
| Stage | What happens |
|---|---|
| History | How long, what changed, stool form, straining, bleeding, weight, family history, and a full medicine list. |
| Examination | Abdominal examination, and usually a rectal examination — which is how impaction, a fissure, a mass and poor sphincter coordination are found. |
| Blood tests and FIT | As above, according to the picture. |
| Referral where indicated | For colonoscopy or specialist assessment where red flags are present or treatment has failed. |
The rectal examination is the step people most want to avoid and the one that most often changes the answer. It takes under a minute.
Frequently Asked Questions
When should I see a doctor about constipation?
If you have a persistent change in bowel habit lasting six weeks or more, particularly aged 50 or over, or any rectal bleeding, unexplained weight loss, an abdominal or rectal lump, unexplained tiredness, narrower stools, night-time symptoms, or constipation that does not respond to treatment. Severe abdominal pain with vomiting and inability to pass wind needs same-day assessment.
What is the FIT test for constipation?
The faecal immunochemical test detects small amounts of blood in a stool sample that are not visible. The referral threshold is 10 micrograms of haemoglobin per gram of faeces or above. A result below the threshold does not rule out a problem, and guidance is explicit that it must not delay referral where clinical concern persists.
Can an underactive thyroid cause constipation?
Yes. Hypothyroidism slows gut transit and is one of the more common treatable causes of constipation that will not settle, usually alongside fatigue, cold intolerance, dry skin and weight gain. A raised blood calcium level is the other blood test result that often turns out to be the explanation.
Why do I strain even though my stools are soft?
That pattern suggests an outlet problem rather than slow transit, where the pelvic floor and anal sphincter do not relax and coordinate properly during defecation. More laxative does not help. Treatment is pelvic floor physiotherapy and biofeedback, so it is worth raising specifically.
Is constipation a sign of bowel cancer?
Usually not, but a persistent change in bowel habit is one of the ways bowel cancer presents, which is why a change lasting six weeks or more, particularly over 50, is assessed rather than treated blindly. Rectal bleeding, weight loss, a lump or iron deficiency anaemia alongside it raise the concern further.
References
- NHS. Constipation. nhs.uk
- National Institute for Health and Care Excellence. Clinical Knowledge Summary: Constipation. cks.nice.org.uk
- Electronic Medicines Compendium. Movicol 13.7g powder for oral solution: Summary of Product Characteristics. medicines.org.uk
- National Institute for Health and Care Excellence. Suspected cancer: recognition and referral (NG12). nice.org.uk
- National Institute for Health and Care Excellence. Quantitative faecal immunochemical testing to guide referral for colorectal cancer in primary care (DG56). nice.org.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. A persistent change in bowel habit, rectal bleeding or unexplained weight loss should always be assessed rather than treated as constipation. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.