IBS and colorectal (colon) cancer can both cause belly pain, diarrhea or constipation, so the worry is understandable. The difference usually lies less in how the pain feels than in what comes with it. IBS is a long-running pattern of pain tied to bowel movements, with changes in stool frequency or form,[12] while colorectal cancer can also cause bleeding, iron-deficiency anemia and weight loss.[4][3] IBS itself is not associated with an increased risk of cancer.[3]
Why do IBS and colon cancer symptoms overlap?
In a US study of 5,075 people diagnosed with colorectal cancer before age 50, four signs in the three months to two years before diagnosis were linked to cancer: abdominal pain, rectal bleeding, diarrhea and iron-deficiency anemia. The more of them a person had, the higher the risk.[4] Two of those, pain and diarrhea, are also everyday IBS symptoms.
The overlap shows up in the numbers. In a Danish study of 57,851 people with a hospital diagnosis of IBS, colon cancer was diagnosed about eight times more often than expected in the first three months after the IBS diagnosis. The researchers put this down to diagnostic confusion: some cancers were first mistaken for IBS.[1] That's why doctors check for warning signs before settling on IBS.
Is it IBS or cancer? The warning signs that need investigation
These features aren't explained by IBS and call for tests, usually including a colonoscopy:[2][6]
| Warning sign | Why it matters |
|---|---|
| Blood in or on the stool, or black, tarry stools | IBS isn't expected to cause bleeding. In a meta-analysis, dark red rectal bleeding was one of only two alarm features that strongly pointed to cancer when present[2][5] |
| Iron-deficiency anemia | An early red flag for colorectal cancer, including in people under 50[4][6] |
| Weight loss you didn't intend | Not a feature of IBS[2][6] |
| A lump in the abdomen | The other alarm feature that strongly pointed to cancer in the meta-analysis[5] |
| Symptoms starting after about 50 | Later onset is an alarm feature in IBS guidelines[6][2] |
| A family history of colorectal cancer | Listed as an alarm feature in US and Asian IBS guidelines[2][6] |
| Symptoms that wake you at night, or fever | Listed among the alarm features to rule out before diagnosing IBS[6] |
UK referral criteria, reproduced in the British IBS guideline, show how age changes the threshold. They call for investigation of suspected colorectal cancer in people aged 50 or over with unexplained rectal bleeding, 60 or over with iron-deficiency anemia or a change in bowel habit, 40 or over with unexplained weight loss and abdominal pain, and anyone with an abdominal or rectal mass. Under 50, rectal bleeding together with pain, a change in bowel habit, weight loss or iron-deficiency anemia is also a reason to refer.[3]
Keep this in perspective. On their own, most alarm features are poor predictors: in the meta-analysis their sensitivity ranged from 5% to 64%,[5] and the American College of Gastroenterology (ACG) notes that alarm features have a low predictive value in people with IBS.[2] They are a reason for tests, not a diagnosis. For the full list, see IBS red flags.
Can IBS cause blood in stool?
IBS itself isn't expected to cause bleeding, so blood should always be checked.[2] Often the source is in the anus. An anal fissure, a small tear, typically causes pain during bowel movements and a little bright red blood on the toilet paper. Internal hemorrhoids cause painless bleeding with bowel movements, and constipation and straining are thought to make them more likely, which is relevant if you have IBS with constipation.[7] Even so, doctors only put bleeding down to hemorrhoids after ruling out other sources,[7] so don't assume.
Can IBS turn into cancer?
No. In the Danish study, the early spike in cancer diagnoses faded quickly. Between one and ten years after an IBS diagnosis, the risk of colorectal cancer was lower than in the general population.[1] A US colonoscopy study found that people with suspected IBS without constipation had no more structural abnormalities than healthy controls. They also had fewer precancerous polyps (adenomas), 7.7% versus 26.1%, and the difference held after adjusting for demographic differences between the groups.[8]
Two caveats. First, IBS doesn't protect you from other conditions, so a new warning sign always deserves a check, whatever your history.[3] Second, IBS isn't the same as inflammatory bowel disease (IBD). Long-standing ulcerative colitis does raise colorectal cancer risk: in a meta-analysis the cumulative risk was 2% after 10 years and 8% after 20 years of colitis.[11] See IBS vs Crohn's disease for how the two are told apart.
When should you get screened for colon cancer?
Colorectal cancer often causes no symptoms until it's advanced, which is why screening matters.[10] The US Preventive Services Task Force (USPSTF) recommends screening for all adults aged 50 to 75 (grade A) and aged 45 to 49 (grade B), and selective screening at 76 to 85 based on your health and past screening.[9] This applies to people at average risk without symptoms. It doesn't cover people with previous colorectal cancer, adenomatous polyps, IBD, or inherited conditions such as Lynch syndrome, who fall outside this recommendation and should ask their doctor about screening.[9]
Having IBS doesn't change this. The ACG stresses staying up to date with screening regardless of IBS symptoms. Its example is a 52-year-old with IBS with diarrhea who has never been screened, who should have a colonoscopy because of their age.[2] Younger adults aren't immune either. About 10.5% of new colorectal cancers occur in people under 50, and incidence in people aged 40 to 49 rose by almost 15% between 2000–2002 and 2014–2016.[9] Being young is no reason to ignore a red flag.
When should you see a doctor?
See a doctor promptly if you have blood in your stool or black stools, iron-deficiency anemia, weight loss you can't explain, a lump in your abdomen, symptoms that started after about 50 or a clear change from your usual pattern, symptoms that wake you at night, or a family history of colorectal cancer or IBD.[2][3][6] Also book a visit if you're 45 or older and not up to date with screening.[9] Get urgent care for heavy bleeding, severe or worsening pain, or feeling faint.
It helps to bring notes: when any bleeding happens and what color it is, your stool type and frequency, when pain occurs, and any change in weight. IBS Clue, an iPhone app coming soon, can log symptoms and Bristol stool types and export them as a PDF report for your doctor; it's a tracking tool and doesn't diagnose. To see what tests to expect, read how IBS is diagnosed.