Red Flag Stool Symptoms: When to See a GP Urgently
Based on NICE NG12 and NHS guidance - Updated June 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 now recommends GPs offer a FIT (faecal immunochemical test) for an unexplained change in bowel habit. A result of 10 micrograms of haemoglobin per gram of faeces or above triggers an urgent suspected-cancer referral. The older 'six weeks, age 60+' rule has been replaced by FIT-guided referral.
NICE NG12 recommends a FIT for adults aged 50+ with unexplained rectal bleeding, and for adults under 50 with rectal bleeding plus abdominal pain or weight loss. A FIT of 10 micrograms of haemoglobin per gram or above prompts urgent referral; a rectal or anal mass is referred without waiting for FIT.
Bowel cancer can cause occult (hidden) blood loss leading to anaemia. NICE NG12 lists unexplained iron-deficiency anaemia as a reason to offer a FIT, and adults aged 60+ with anaemia even without iron deficiency should also be tested.
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. Since NICE introduced the FIT-guided colorectal pathway in 2023, GPs usually arrange a FIT (faecal immunochemical test) first to decide who needs urgent referral; a result of 10 micrograms of haemoglobin per gram of faeces or above is the threshold.
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 (colorectal FIT pathway introduced 2023), GPs now use a faecal immunochemical test (FIT) to guide urgent referral. A FIT is offered to adults with an unexplained change in bowel habit, iron-deficiency anaemia or an abdominal mass; to adults aged 50 and over with unexplained rectal bleeding, abdominal pain or weight loss; to adults under 50 with rectal bleeding plus abdominal pain or weight loss; and to adults aged 60 and over with anaemia even without iron deficiency. A FIT result of 10 micrograms of haemoglobin per gram of faeces or above prompts an urgent suspected-cancer (2-week-wait) referral. People with a rectal or anal mass, or unexplained anal ulceration, are referred without needing a FIT first.
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 June 2026. Sources: NICE NG12 (Suspected cancer: recognition and referral, last updated April 2026; FIT-guided colorectal pathway from 2023). NHS UK bowel cancer guidance. Bowel Cancer UK clinical guidance.