5 Warning Signs of Colorectal Cancer You Shouldn’t Ignore

Colorectal cancer is one of the most common cancers in the United States and one of the most preventable when detected early. Routine screening is now recommended beginning at age 45 for adults at average risk, and earlier for those with certain risk factors, such as family history or inherited conditions. Still, many people delay seeking care because early symptoms can be vague, subtle, or easily attributed to everyday digestive issues.

According to Samir Gupta, MD, gastroenterologist at UC San Diego Health and a nationally recognized expert in colorectal cancer screening and prevention, being aware of potential warning signs is just as important as following age- and risk-based screening guidelines.

"Most people who experience these symptoms do not have colorectal cancer," Gupta says. "But if symptoms are new, persistent, or unexplained after an initial medical evaluation, it's important to take the next step to rule it out. Importantly, people should not wait for symptoms to get screened — colorectal cancer often develops silently."

Listening to your body and acting when something doesn't feel right can make a meaningful difference. If you're experiencing ongoing digestive symptoms, don't ignore them. Early evaluation can lead to better outcomes.

Gupta shares five warning signs of colorectal cancer you shouldn't ignore.

1. Blood in the Stool

Blood in the stool, or on toilet paper after a bowel movement, is often caused by common conditions like hemorrhoids. However, it can also be a sign of colorectal cancer, particularly when bleeding is consistent. Blood may appear bright red or darker and tar-like, depending on where bleeding occurs in the colon or rectum. Any ongoing or unexplained bleeding should be evaluated by a health care provider.

At UC San Diego Health, gastroenterology specialists use advanced diagnostic tools, including colonoscopy, to identify the cause and ensure serious conditions are not overlooked.

2. Low Iron levels

Low iron levels, especially in adults without an obvious cause, may be a sign of slow or hidden blood loss in the digestive system. In some cases, colorectal tumors can bleed internally without visible blood in the stool.

Symptoms may include fatigue, weakness, shortness of breath, or dizziness. If blood tests reveal low iron levels and initial treatment does not resolve the issue, further evaluation of the colon may be recommended.

"Unexplained iron-deficiency anemia should always prompt careful follow-up," Gupta says.

3. A Change in Bowel Habits

Ongoing change in bowel habits lasting more than a few weeks can be a warning sign. This may include:

• More frequent bowel movements
• Less frequent bowel movements
• Ongoing diarrhea or constipation
• Narrow or pencil-thin stools

While diet changes, stress, and medications can affect digestion, symptoms that persist without a clear explanation should be discussed with a doctor.

4. Persistent Abdominal Pain or Cramping

Occasional abdominal discomfort is common, but new or persistent abdominal pain, especially pain that worsens over time or does not respond to usual treatments, can be concerning. Colorectal cancer may cause cramping, bloating, or a feeling of fullness that doesn't go away. Pain that interferes with daily life or appears alongside other symptoms on this list should be evaluated.

5. Unintentional Weight Loss

Losing weight without trying, particularly when it occurs alongside digestive symptoms, can be a sign that something is wrong. Unintentional weight loss may occur when cancer affects appetite, digestion, or the body's ability to absorb nutrients. Any unexplained weight loss should prompt a conversation with a health care provider, especially if other gastrointestinal symptoms are present.

Most people who experience these symptoms do not have colorectal cancer. However, Gupta emphasizes that if symptoms remain unexplained after seeing a doctor and completing initial tests or treatments, patients should ask whether a colonoscopy is needed to rule it out.

Early screening can prevent cancer or catch it when it's most treatable

Even in the absence of symptoms, screening remains the most effective way to detect colorectal cancer early or prevent it altogether by identifying and removing precancerous polyps. The American Cancer Society recommends screening for adults beginning at age 45 and continuing through age 75. Those with a family history of colorectal cancer, a personal history of inflammatory bowel disease, or inherited genetic syndromes may need to begin screening earlier and undergo testing more frequently.

At UC San Diego Health, patients have access to comprehensive colorectal cancer screening and care, combining nationally recognized clinical expertise with leading-edge research and a patient-centered approach. As part of the region's only academic medical center, UC San Diego Health physicians are also helping shape national screening strategies to improve early detection and save lives.

For those at higher risk, specialized screening and care are available

For individuals who may be at increased risk due to family history or inherited genetic conditions, such as Lynch syndrome or familial adenomatous polyposis, specialized care is available through the Colorectal Cancer High-Risk Program at UC San Diego Health.

About 5–10% of colorectal cancers are caused by inherited gene changes that run in families. Identifying these risks early can help people start screening sooner and take steps to prevent cancer altogether. The program provides genetic risk assessment and testing, personalized screening plans that often begin earlier and occur more frequently and coordinated care with colorectal cancer specialists. Genetic counseling is also available to help family members understand and manage their own risk, encouraging patients and families to take proactive steps toward early detection and prevention.