Is It Normal to Have Thin Stools?

Medically reviewed by Moshe Barnajian, MD, FASCRS, colorectal surgeon at Surgery Group LA · Reviewed September 25, 2026

An occasional thin or narrow stool is usually nothing to worry about. Stool shape changes with what you eat, how much water you drink, and how hard you strain. If your stools stay pencil-thin for more than a week or two, or you also notice bleeding, belly pain or weight loss you can't explain, it's worth getting checked, because a persistent change can sometimes mean something is narrowing the colon or rectum (Mayo Clinic).

What counts as a "thin" stool?

Most people's stools vary from day to day. Patients describe thin stools as pencil-thin, ribbon-like, flat or stringy. A normal stool followed by a thinner one at the end of the same bowel movement is common and usually reflects how the last part of the stool passed, not a problem in the colon. What matters more is a change that sticks: stools that are consistently narrower than usual over several weeks.

4 possible explanations for thin stools

1. Constipation and diet. When stool is hard and dry, you tend to strain, and the stool can come out in smaller, narrower pieces. Low fiber, not enough fluids and less activity all play a part. Some people also notice thin or loose stools after taking a laxative, as the bowel empties more quickly than usual. This kind of change usually improves once your bowel habits settle.

2. Irritable bowel syndrome (IBS). IBS can change the size and consistency of stools. It often comes with gas, bloating, cramping, and alternating constipation and diarrhea (Mayo Clinic). If you often have thin stools along with gas or bloating, IBS or another functional bowel problem is a common explanation, but it should be diagnosed by a doctor rather than assumed.

3. Colorectal cancer or a large polyp. Is the idea that thin stools mean colon cancer a myth? Not entirely. Most thin stools are not cancer. But a growth in the lower colon or rectum can narrow the passage and change the shape of the stool, and "narrowing of the stool" is one of the recognized warning signs of colorectal cancer (American Cancer Society). Other warning signs include rectal bleeding, dark or black stools, a feeling that you still need to go after a bowel movement, belly pain, fatigue and unintended weight loss. Many of these symptoms can also come from hemorrhoids, infection or IBS. That's why an exam, rather than guessing, is the way to sort it out (American Cancer Society).

4. Anal cancer (rare). Less commonly, a growth near the anus can change stool shape. It may also cause bleeding, pain, itching, discharge or a lump near the anus. Anal cancer is uncommon and is often linked to HPV infection.

Other conditions, such as scarring or narrowing of the anal canal or large hemorrhoids, can also affect stool shape. A colorectal surgeon can usually tell these apart with a short exam.

What you can do at home

If your stools have only just changed and you feel otherwise well, give simple changes a week or two:

• Add fiber gradually from fruits, vegetables, beans and whole grains, or take a fiber supplement.
• Drink more water through the day.
• Stay active; even regular walking helps bowel movements.
• Go when you feel the urge, and avoid straining or sitting on the toilet for long periods.

When to see a doctor about thin stools

Make an appointment if:

• your stools stay thin or narrow for more than 1–2 weeks (Mayo Clinic);
• the change comes with blood in or on the stool, dark or black stools, ongoing belly pain, fatigue or weight loss you can't explain;
• you're 45 or older and haven't had colorectal cancer screening. The U.S. Preventive Services Task Force recommends that average-risk adults start screening at 45 (USPSTF).

Seek care right away if a change in your bowel movements comes with rectal bleeding or severe belly pain (Mayo Clinic), or if you can't pass stool or gas and your belly is swollen or you're vomiting.

How we evaluate a change in stool shape

At your visit, we'll ask how long the change has lasted, what else you've noticed, and about your family history. We'll usually do a gentle rectal exam. Depending on what we find and your age and risk, we may recommend a colonoscopy, which lets us look for polyps or other causes and remove polyps during the same procedure. You can read more about colorectal cancer screening options and colon cancer care at Surgery Group LA.

Talk to a colorectal surgeon

If something has changed and you'd rather not keep wondering, we're happy to take a look. Our colorectal surgeons see patients at the Cedars-Sinai Medical Office Towers, 8635 W 3rd St, Suite 880W, Los Angeles. Call (310) 861-7493 or request an appointment.

This article is for general information and is not a substitute for medical advice.

Sources: Mayo Clinic — Narrow stools: Should I be concerned? · American Cancer Society — Colorectal Cancer Signs and Symptoms · USPSTF — Colorectal Cancer Screening