Red Flag Stool Symptoms: When to See a GP Urgently
Based on NICE NG12 and NHS guidance - Updated April 2026
This page describes symptoms that require GP assessment. It does not replace medical advice.
If you have blood in your stool, black tarry stool with dizziness or collapse, or severe abdominal pain, call 999 or go to A&E. Do not wait for a GP appointment.
Red Flags - Explained by Urgency
This combination indicates a significant upper gastrointestinal bleed. Immediate emergency care is required. Do not drive yourself to hospital.
May indicate bowel perforation, mesenteric ischaemia, or other surgical emergency. 999.
May indicate severe infectious colitis, toxic megacolon in UC, or ischaemic colitis. Emergency presentation required.
Melaena indicates an upper GI bleed until proven otherwise. A digested blood stool has a distinctive tar-like consistency and offensive smell, distinct from iron- or bismuth-blackened stool. Same-day GP or A&E review.
Small amounts on paper from haemorrhoids are common. A large amount of red blood (in the bowl or soaking through stool) warrants same-day assessment.
While haemorrhoids are the most common cause at any age, rectal bleeding in younger adults without a known cause (IBD, haemorrhoids) should be assessed promptly.
Even if you believe haemorrhoids are the cause, persistent rectal bleeding should be formally assessed. Haemorrhoids and bowel cancer can coexist.
A small amount of mucus is normal. Visible mucus mixed through stool suggests IBD, infection, or bowel cancer and warrants GP review.
NICE NG12 criterion: a change in bowel habit (more frequent or looser stools) in adults 60+ for 6+ weeks without rectal bleeding is a 2-week-wait referral criterion.
NICE NG12: rectal bleeding with looser or more frequent stools persisting 6+ weeks in adults 40+ is a 2-week-wait urgent cancer referral criterion.
Bowel cancer can cause occult (hidden) blood loss leading to anaemia. Unexplained iron deficiency anaemia in adults warrants bowel investigation regardless of symptoms.
Not an emergency, but warrants GP review to exclude treatable causes (IBS, diet, medication, thyroid).
Diarrhoea that wakes you from sleep is organic (caused by a physical condition) - IBS does not typically cause nocturnal symptoms. Book a GP appointment.
Unintentional weight loss (more than 5% in 3 months) combined with any change in bowel habit should prompt GP review.
If a first-degree relative (parent, sibling, child) has had bowel cancer under 50, your screening threshold should be lower. Discuss with your GP.
The NICE 2-Week-Wait: What It Means
The NHS operates an "urgent suspected cancer referral" pathway - commonly called the 2-week-wait or 2WW - under NICE guideline NG12 (Suspected cancer: recognition and referral). When a GP refers you on this pathway, you should be seen by a specialist within 14 days.
A 2-week-wait referral does not mean you have cancer. The vast majority of people investigated on this pathway are found to have benign conditions (haemorrhoids, diverticular disease, IBD, polyps). The pathway exists to ensure that those who do have bowel cancer are diagnosed and treated as quickly as possible, when treatment is most effective.
If your GP refers you urgently, accept the referral. If your GP dismisses symptoms that you are concerned about and that match the criteria above, you have the right to seek a second opinion. Bowel Cancer UK operates a free helpline and can advise on accessing investigation.
Black Stool: Melaena vs Iron/Bismuth
Melaena (medical emergency)
- Very dark black, tar-like, sticky consistency
- Strongly offensive, distinctive smell
- May be liquid or semi-liquid
- Often accompanied by nausea, dizziness
- Indicates upper GI bleed (oesophagus, stomach, duodenum)
Iron / Bismuth staining (usually benign)
- Dark green-black or very dark brown
- Firm, formed stool
- No distinctive offensive odour beyond normal
- Common with ferrous sulfate, ferric supplements
- Common with Pepto-Bismol (bismuth subsalicylate)
If you have recently started iron supplements or taken Pepto-Bismol and notice black stool, this is likely medication-related. If you are not taking either and notice black tarry stool, treat it as melaena and seek same-day care. If uncertain, seek care - it is always better to be assessed and reassured.
FAQ
What is melaena and is it an emergency?
Melaena is black, tarry, foul-smelling stool caused by digested blood from an upper gastrointestinal bleed (oesophagus, stomach, or duodenum). It is a medical emergency if accompanied by dizziness, rapid heartbeat, collapse, or vomiting blood. Call 999 or go to A&E immediately. Black stool from iron supplements or bismuth is a different appearance - firm, dark, not tarry or foul-smelling - but any uncertainty about black stool should be assessed by a GP the same day.
When should I be referred urgently for bowel cancer investigation?
Under NICE guideline NG12, GPs should refer adults aged 40+ on a 2-week-wait pathway if they have rectal bleeding with a change in bowel habit (more frequent or looser stools) lasting 6+ weeks; or adults 60+ with rectal bleeding (without an anal cause) for 6+ weeks; or adults 60+ with a change in bowel habit without rectal bleeding lasting 6+ weeks. GPs should also consider 2-week-wait referral for adults of any age with unexplained rectal mass, or unexplained iron-deficiency anaemia.
Can I have bowel cancer without any symptoms?
Yes. Early-stage bowel cancer frequently causes no symptoms, which is why the NHS Bowel Cancer Screening Programme invites adults aged 50-74 every two years. The home faecal immunochemical test (FIT test) detects tiny amounts of blood invisible to the naked eye. Return your FIT test when invited. If you are in a higher-risk group (family history of bowel cancer, personal history of polyps, IBD), discuss earlier screening with your GP.
Updated April 2026. Sources: NICE NG12 (Suspected cancer: recognition and referral). NHS UK bowel cancer guidance. Bowel Cancer UK clinical guidance.