A Common Sign of Colorectal Cancer That Young Adults Might Dismiss

AMID A WORRYING RISE in colorectal cancer in people under 50, researchers presented data at a recent conference that drives home important medical advice for young people:

Don’t ignore blood in your stool.

Research presented at the American College of Surgeons Clinical Congress, held Oct. 4 to 7, 2025, in Chicago, showed that rectal bleeding was the top indicator of colorectal cancer in people under 50 who had colonoscopies. The study analyzed results from 443 people under 50 who received colonoscopies at the University of Louisville Health System between 2021 and 2023. It found that rectal bleeding, which can appear in different ways such as blood found in stool or on toilet paper, was more predictive of colorectal cancer than family history of the disease or smoking.

Of people included, 7 in 10 underwent a colonoscopy because they were experiencing a medical symptom. About 44% of people in the study were ultimately diagnosed with colorectal cancer. Among those with colorectal cancer, about 4 in 10 reported bleeding. In comparison, 3 in 10 had a family history of the disease, which was the next highest risk factor.

Other studies have isolated rectal bleeding as a top predictor of a colorectal cancer diagnosis, says Joshua Demb, a health science researcher at the University of California, San Diego, and the San Diego VA Healthcare System who was not involved in the research. He and colleagues did a review that examined red flags for early-onset colorectal cancer, cases diagnosed before age 50. That review, published May 24, 2024, in JAMA Network Open, found about 45% of those diagnosed with colorectal cancer reported rectal bleeding along with 40% who reported abdominal pain, and 27% who noted a change in their bowel habits.

The review also found people who had a colonoscopy and received a colorectal cancer diagnosis waited on average four to six months between when symptoms first appeared and the diagnosis of colorectal cancer. That lines up with previous research that found delays between symptom presentation and colorectal cancer diagnosis are up to 40% longer in young people than in people diagnosed after age 50 and adds to concerns the disease may advance to more perilous stages during that crucial window, Demb says.

“We’re often seeing early-onset patients … presenting with stage III or IV cancer,” Demb says. “[Perhaps] it could have actually been an earlier stage, had it been detected earlier.”

When to Tell Your Doctor About Blood in Stool

So, what should one look out for?

It’s important to know that blood in the stool can look different from individual to individual, says Kimmie Ng, a medical oncologist and director of the Young-Onset Colorectal Cancer Center at Dana-Farber Cancer Institute in Boston. Darker colored stool can be a sign of bleeding higher up in the digestive track. Bright red blood, including in the toilet bowl or on toilet paper, often comes from lower in the rectum and could indicate benign conditions such as hemorrhoids or an anal fissure.

Depending on the characteristics of the bleeding and the presence or absence of other symptoms that are associated with colorectal cancer, such as abdominal pain, changes in bowel habits or unexplained weight loss, a doctor may recommend a colonoscopy or start with other tests. These can include physical evaluations, X-rays, and blood and stool tests identify the cause of gastrointestinal bleeding, according to the National Institutes of Health.

But there’s no way for a person at home to tell definitively what caused the bleeding, and Ng urges people to be proactive.

“The take home is, if there’s any blood, people should see their doctor about it,” Ng says. “Certainly if it’s not going away or it’s getting worse, it does need to be worked up further.”

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