Rectal bleeding in adults under 50 is a strong warning sign of colorectal cancer, a new study reports, finding the symptom increases the odds of an early-onset colorectal cancer diagnosis by 8.5 times.
Researchers analyzed 443 patients under 50 who underwent colonoscopy at the University of Louisville Health System between 2021 and 2023. Of those, 195 were diagnosed with early-onset colorectal cancer and 248 had clear colonoscopy results. The team reported that 88% of the young cancer patients underwent colonoscopy because of symptoms rather than routine screening, and 70% had no family history of colorectal cancer. People who had smoked in the past were more than twice as likely to develop early-onset colorectal cancer as those who had never smoked. The study was presented at the American College of Surgeons Clinical Congress 2025 in Chicago and has not yet been published in a peer-reviewed journal.
Authors and outside experts say the findings mean both patients and clinicians should treat rectal bleeding in younger adults seriously. “Many of the early-onset colorectal cancers that I see have no family history,” said Sandra Kavalukas, MD, colorectal surgeon at the University of Louisville School of Medicine and a senior author of the study. “If you have a person below the screening age with rectal bleeding, you should seriously consider a colonoscopy.”
Other experts interviewed agreed. “Young people with rectal bleeding should get a colonoscopy,” said Alan Venook, MD, professor of medical oncology at UCSF. Alan Bilchik, MD, a surgical oncologist at Providence Saint John’s Cancer Institute, said clinicians should no longer assume rectal bleeding in younger adults is caused by hemorrhoids. Robert Smith, MD, senior vice president of early cancer detection science at the American Cancer Society, said doctors often downplay symptoms in younger patients and that persistent rectal bleeding is not normal and should be investigated. Nilesh Vora, MD, medical director of the MemorialCare Todd Cancer Institute, urged people under 50 not to ignore symptoms.
The American Cancer Society estimates there will be more than 150,000 new colorectal cancer cases in the United States this year—more than 100,000 colon cancer cases and slightly fewer than 50,000 rectal cancer cases—with diagnoses split almost evenly between males and females. Colorectal cancer is a leading cause of cancer deaths in the U.S.: it ranks among the top causes for men and women and is expected to cause about 53,000 deaths this year. While overall colorectal cancer incidence has been declining since the mid-1980s—largely due to screening and lifestyle changes—the trend is different in younger adults. Between 2012 and 2021, the rate of colorectal cancer diagnosis increased more than 2% per year in people under 50, and the death rate has been rising slightly in this group. The National Cancer Institute reports colon cancer is the leading cause of cancer-related death in adults ages 20 to 49 in the United States. Smith noted that people born around 1990 have about twice the risk of colorectal cancer compared with those born around 1950.
Researchers do not yet know the exact causes of the increase in early-onset colorectal cancer. Suspected factors include diet, lack of physical activity, obesity, overuse of antibiotics, systemic inflammation, and changes in intestinal bacteria. Venook suggested exposure to certain gut bacteria in childhood could lead to cancer decades later.
Experts stress that colorectal cancer is curable when caught early, which is why screening matters. Current recommendations call for men and women to begin colorectal cancer screening at age 45, with earlier screening for people with a family history of the disease or conditions such as inflammatory bowel disease. Colonoscopy is generally considered the best screening test and is typically repeated every 10 years for people without a family history and with no polyps found; other options include at-home stool kits. Besides rectal bleeding, symptoms of colorectal cancer can include changes in bowel habits, diarrhea, constipation, cramping or abdominal pain, and unexplained weight loss. Family history of colorectal cancer should be discussed with a doctor, especially if relatives were diagnosed young.
Ways to lower colorectal cancer risk include eating a high-fiber, lower-fat diet with fewer red and processed meats and more poultry, fish, legumes, fruits, vegetables, and whole grains; engaging in regular physical activity (the American Institute for Cancer Research recommends about 2.5 hours of moderate activity weekly); reducing alcohol intake; not smoking; and maintaining a healthy weight. Experts also recommend adopting healthy habits early in life to reduce long-term risk.
