5 Common Causes of Lower GI Bleeding: When to Worry and When to See a Doctor

5 Common Causes of Lower GI Bleeding: When to Worry and When to See a Doctor

Lower Gastrointestinal (LGI) bleeding originates anywhere from the small intestine down to the colon, rectum, or anus.

Whilst seeing blood in your stool can be a heart-stopping moment, the shade of red and the presence of other symptoms are the primary tools doctors use to pinpoint the source.

Because LGI bleeding doesn’t come into contact with stomach acid, it rarely appears as “coffee grounds” or “tar.” Instead, it usually presents as various shades of red. Here are the five most common causes.

1. Diverticular Disease

Diverticulosis is a condition where small, bulging pouches (diverticula) develop in the lining of the digestive tract, most commonly in the lower part of the large intestine (colon).

• The Cause: While these pouches are often harmless, a small blood vessel located within a pouch can thin out and eventually rupture.

• The Sign: This is the leading cause of brisk, painless bleeding. You may notice a sudden, large amount of bright red or maroon blood passed during a bowel movement.

2. Haemorrhoids and Anal Fissures

These are the most frequent causes of visible LGI bleeding and, while usually not life-threatening, they can be incredibly uncomfortable.

• The Cause: Haemorrhoids are swollen veins in the rectum or anus (essentially “varicose veins” of the bottom). Anal fissures are small tears in the lining of the anus, often caused by straining or passing hard stools.

• The Sign: You will typically see bright red blood on the toilet tissue or dripping into the bowl. Fissures are usually accompanied by significant sharp pain during or after a bowel movement.

3. Inflammatory Bowel Disease (IBD)

Chronic conditions like Ulcerative Colitis and Crohn’s Disease involve the immune system attacking the lining of the gut, causing severe inflammation and sores.

• The Cause: The inflammation erodes the intestinal lining, leading to open sores (ulcers) that bleed.

• The Sign: Bleeding is often mixed with stool and is frequently accompanied by bloody diarrhoea, mucus, abdominal cramping, and an urgent need to use the bathroom.

4. Angiodysplasia

This condition involves swollen, fragile blood vessels in the GI tract, most commonly in the right side of the colon.

• The Cause: These “vascular malformations” become more common as we age and the blood vessels in the colon wall weaken and enlarge.

• The Sign: This usually causes painless, intermittent bleeding. The blood may appear maroon or bright red, or it may be “occult” (hidden), only appearing on a laboratory stool test.

5. Colorectal Polyps and Cancer

Polyps are small growths on the lining of the colon. While many are benign, some can develop into cancer over time.

• The Cause: As tumours or large polyps grow, they become fragile and bleed easily when stool passes over them.

• The Sign: Bleeding may be dark red or maroon and mixed into the stool.

• Warning Signs: A persistent change in bowel habits (e.g., lasting diarrhoea or constipation for more than 3 weeks), “pencil-thin” stools, and unexplained weight loss.

The Colour Guide to Lower GI Bleeding

The closer the bleeding source is to the “exit,” the brighter the red will be.

When to Worry: The Emergency “Red Flags”

Lower GI bleeding can sometimes be massive. Seek immediate medical attention if you experience:

• Large volumes of blood that do not stop.

• Dizziness, fainting, or confusion (signs of shock).

• Cold, clammy skin and a rapid pulse.

• Severe, rigid abdominal pain or bloating.

When to See a Doctor (Non-Emergency)

Even if you feel fine, you should book an appointment if you notice:

• Any blood in the stool that persists for more than a day or two.

• A change in how often you go to the toilet lasting more than three weeks.

• Symptoms of Iron-Deficiency Anaemia: Feeling unusually tired, pale, or short of breath.

• A strong family history of bowel cancer or polyps.

Next Steps in Diagnosis

For lower GI bleeding, the “gold standard” investigation is a Colonoscopy. A specialist uses a long, flexible tube with a camera to examine the entire length of the colon.

Similar to a Gastroscopy, this procedure allows doctors to take biopsies or even remove bleeding polyps on the spot.