Reasoning GP · Algorithm · 1-page summary
Constipation in adults
For: adults ≥18 with new-onset, chronic or opioid-induced constipation in primary care. Not for: children (constipation in children); rectal bleeding as the main problem (rectal bleeding). If IBS-C fits better, manage as IBS.
Core rule: screen for obstruction, red flags, drug causes and impaction before any laxative. An unexplained change in bowel habit needs FIT at any age, and a FIT below 10 µg Hb/g does not close the case if concern persists.
0Stop — emergency — before any laxative
- Suspected obstruction (absolute constipation, vomiting, distension, no flatus), perforation, volvulus, peritonitis, shock or severe uncontrolled pain → Same-day hospital; 999 if unstable or cannot be safely transported. No oral disimpaction meanwhile.
- New urinary retention, saddle anaesthesia, bilateral leg weakness or rapidly progressive deficit (cauda equina) → Immediate emergency. Do not delay for a primary-care neurological examination.
- Clozapine with constipation plus pain, distension, vomiting, overflow diarrhoea or systemic illness → Same-day urgent. Do not wait several days (ileus, ischaemia, death).
1Triage — what kind of constipation?
| Picture | Next step |
|---|---|
| Short-lived, obvious cause | Treat; FIT not needed. |
| Unexplained change | New looser stools or constipation, any age → FIT (block 2). |
| Impaction ± overflow | Exclude obstruction, then treat. Same-day review if pain, distension, vomiting, dehydration, frailty or no response. |
| Chronic, no red flags | Rome IV (≥2 criteria, ≥3 months) → classify. |
2Screen — NICE NG12, updated April 2026
- Offer FIT: change in habit or iron-deficiency anaemia (any age); abdominal mass; weight loss + pain ≥40; bleeding + pain or weight loss <50; unexplained bleeding, pain or weight loss ≥50; anaemia ≥60 → ≥10 µg Hb/g → Suspected cancer pathway
- Rectal mass, or unexplained anal mass or ulcer → consider a suspected cancer pathway referral without waiting for FIT. A negative, unavailable or unreturned FIT, or an abdominal mass, must not delay referral when concern is strong.
- Unexplained IDA: coeliac serology + FIT; men and postmenopausal women need upper and lower GI assessment even if FIT is negative.
- Severe bleeding or haemodynamic compromise → Emergency, not FIT. Lynch / FAP history → genetic referral per local pathway.
3Assess
☐ Bristol type, frequency, straining, blood, weight, obstetric history ☐ Drugs: opioids, iron, CCBs, TCAs, antipsychotics, antihistamines, anticholinergics — ask about clozapine ☐ Abdomen, perianal inspection ☐ PR with consent if new, unexplained or refractory, impaction, bleeding, mass or neurological disease
Bloods by context only (FBC + ferritin, U&E/calcium, TSH, HbA1c, coeliac). Not routine: AXR, calprotectin for isolated constipation, colonoscopy without criteria.
4Classify — drives treatment
- Functional → laxative ladder. Outlet symptoms → refer, don't just escalate laxatives.
- Drug-induced → review indication, dose and route. Metabolic — hypothyroidism, hypercalcaemia, hypokalaemia, diabetes, uraemia → treat the cause first.
- Neurological (Parkinson's, MS, cord) — often refractory. Structural — fissure, prolapse, rectocele, cancer.
- IBS-C — pain relieved by defaecation → IBS pathway.
- Uncertain — say so; 1-week bowel diary, review at 2–4 weeks, re-screen block 2.
5Treat today — doses: NICE/SPC, BNF, local formulary
- Functional: ispaghula if small hard stools with adequate fluid; macrogol 1–3 sachets/day if hard stools, loading or bulk not tolerated (lactulose 15 ml BD alternative) → add senna 7.5–15 mg or bisacodyl 5–10 mg at night.
- Opioid-induced: regular stimulant ± osmotic from the start of a strong opioid; not bulk-forming alone. PAMORA only after appropriate laxatives (NICE TA345/TA651).
- Impaction: exclude obstruction → high-dose macrogol per SPC (check hydration, renal function, heart failure) ± rectal treatment; never a stimulant alone.
- Clozapine: regular stimulant + osmotic/softener; bowel chart. Always: gradual fibre, fluid individualised (heart failure, CKD), activity, respond to the urge, footstool.
6Review and refer routinely
- Review at 2–4 weeks (sooner if impaction, clozapine, frailty); no better on step 3 by 6–8 weeks → consider referral.
- Refractory by clinical need (severe, recurrent impaction, neurological, nutritional impact) → gastro/colorectal; no fixed 6-month wait.
- Defaecatory dysfunction → anorectal physiology; biofeedback (continence nurse or physio).
- Prucalopride: women meeting NICE TA211 only, by an experienced clinician.
7Never reassure by
- A negative or unreturned FIT when concern persists.
- Bowel sounds, or an empty rectum on PR.
- Loose stool — it may be overflow around impaction.
- Age alone: new constipation in an older adult needs assessment.
- An old "functional" label when new red flags appear.
- Clozapine constipation that "can wait a few days".
8Safety-net — say it, then document it
999 “Vomiting, a swollen tummy, no wind, or severe pain with a rigid tummy or fever — same-day hospital assessment; 999 if you collapse, are very unwell or can't travel safely. Can't pass urine, numbness between the legs or leg weakness — emergency help now.” Same-day GP “Any new bleeding from the bottom, weight loss, a lump, worsening pain, or impaction not clearing after 3 days of treatment.” On clozapine: pain, swelling, vomiting or leaking stool → same-day medical assessment. New red flags at any point → back to block 0.
✓Safe to leave primary care only if — all five, documented
No block 0–2 row applies
Drugs reviewed, incl. opioids and clozapine
Impaction excluded or treated; PR where indicated
FIT or bloods by criteria; result owner named
Review in 2–4 weeks booked; safety-net given
Summary of the full Reasoning GP constipation algorithm, which holds the detail, examination, ladder and evidence. Sources: NICE NG12 (updated April 2026), HTG690, CG140, TA345, TA651, TA211, CG61; BSG IDA 2021; BSG/ACPGBI FIT in IDA 2025; SPS clozapine; MHRA; local formulary. Clinical reference — follow local pathways and check doses in the current BNF and SPC.