Red flag symptom that is the 'strongest predictor of colorectal cancer' in patients under 50

Warning: This article contains discussion of cancer which some readers may find distressing.

New research has identified the symptom which is the biggest indicator of colorectal cancer in the under 50s, and it is a very serious thing that health experts take as a sign to see a doctor urgently.

Researchers at the University of Louisville School of Medicine studied 443 patients under the age of 50 who had undergone a colonoscopy, finding that 44 percent of them were diagnosed with early-onset colorectal cancer.

The likelihood of someone under 50 being diagnosed with colorectal cancer was 8.5 times higher if among their symptoms which resulted in them getting a colonoscopy was rectal bleeding.

"Many of the early-onset colorectal cancers that I see have no family history,” said senior author and colorectal surgeon Dr Sandra Kavalukas.

"This research lends support to the question of who does or doesn’t warrant a colonoscopy: if you have a person below the screening age with rectal bleeding, you should seriously consider a colonoscopy."

Colorectal cancer is growing among the under 50 age group (Getty Stock Photo)

The researchers found that 88 percent of patients who went on to be diagnosed with colorectal cancer had undergone a colonoscopy because they were showing symptoms like rectal bleeding.

Of those patients who were part of the study just 13 percent had genetic markers which indicated a hereditary history of colorectal cancer, having a family history of cancer still increased the risk of someone getting it.

“If they’re 35 and they come in with rectal pain, they probably don’t need a colonoscopy. But if they come in with a bleeding complaint, they are 8.5 times more likely to have a colorectal cancer," the doctor warned.

Colorectal cancer is on the rise among young people, which in this context means people under 50, a 2026 study by the American Cancer Society (ACS) found that between 1990 and 2023 cancer mortality among under-50s dropped by 44 percent but since 2005 cases of colorectal cancer have increased by 1.1 percent each year.

For Americans it's risen from the fifth to the first deadliest cancer among the under 50 age group.

In the UK it's the fourth most common cancer and and the second most common cause of death from cancer.

Researchers have warned that rectal bleeding, often spotted by blood in poo, was a sign that someone was 8.5 times more likely to develop early onset colorectal cancer (Getty Stock Photo)

The signs of rectal bleeding

According to the NHS if you're bleeding rectally then you should see a doctor, and if your poo is black or dark red then you should call for an urgent appointment.

They warn that if you're bleeding without stopping or there's lots of blood when you poo then you need to call 999 and get yourself seen to as a matter of urgency.

The NHS says the warning signs to look out for are:

• Blood on your toilet paper
• Red streaks on the outside of your poo
• Pink water in the toilet bowl
• Blood in your poo or bloody diarrhoea
• Very dark poo, a potential sign of blood mixed in poo

If you’ve been affected by any of these issues and want to speak to someone in confidence, contact Macmillan’s Cancer Support Line on 0808 808 00 00, 8am–8pm seven days a week.