Bloody Diarrhea: Causes and When to Worry

What bloody diarrhea means

Blood in diarrhea usually means the lining of the colon or rectum is inflamed or broken, a condition called colitis. When this comes with fever, cramps and frequent small stools with mucus, it is often called dysentery. The color gives clues:

• Bright red blood: usually from the colon, rectum or anus
• Maroon or dark red: higher in the colon or the small bowel
• Black, tarry stool (melena): bleeding from the stomach or upper bowel, a different problem that needs urgent care

Red or black stool can also come from beets, red food coloring, iron tablets and bismuth (Pepto-Bismol), which cause discoloration but not true bleeding.

Causes of bloody diarrhea

Bacterial infections (most sudden cases)

See campylobacter infection and salmonellosis. Most bacterial infections settle within a week.

Other infections

• C. difficile colitis: watery, sometimes bloody diarrhea during or after antibiotics, or after a hospital stay.
• Amebic dysentery: from the parasite Entamoeba histolytica, mainly after travel to areas with poor sanitation. Symptoms build over 1 to 3 weeks.
• Cytomegalovirus colitis: in people with weak immune systems.

Inflammatory bowel disease

Ulcerative colitis typically causes bloody diarrhea with mucus, urgency and tenesmus (a constant feeling of needing to go) that lasts for weeks. Crohn's disease can also cause bloody diarrhea, often with pain, weight loss and mouth ulcers. Both usually start between ages 15 and 35, but can begin at any age.

Ischemic colitis

Reduced blood flow to part of the colon causes sudden cramping pain, usually on the left, followed within 24 hours by bloody diarrhea. It mainly affects people over 60, especially with heart disease, vascular disease or dehydration. See ischemic colitis.

Other causes

• radiation colitis after pelvic radiotherapy
• medicines, including NSAIDs and some cancer immunotherapies
• colorectal cancer, which more often causes a change in bowel habits with blood than sudden bloody diarrhea
• hemorrhoids or an anal fissure bleeding during a bout of ordinary diarrhea, where blood is on the paper or the stool surface rather than mixed in
• intussusception in babies and toddlers: "redcurrant jelly" stools with episodes of severe crying and drawing up the legs

Hemolytic uremic syndrome: the danger to watch for

About 5 to 15% of children infected with E. coli O157 develop hemolytic uremic syndrome (HUS), usually about a week after the diarrhea started, often as it is getting better. Red blood cells break down and the kidneys fail. Warning signs are passing little urine, pale skin, tiredness, swelling and small unexplained bruises. Antibiotics and antidiarrheal drugs can increase the risk, which is one reason not to take them for bloody diarrhea without testing.

How doctors find the cause

A doctor will ask about food, travel, contacts with similar illness, recent antibiotics or hospital stays, how long it has lasted, and other symptoms such as fever, weight loss and joint pain. Tests may include:

• Stool tests: a multiplex PCR panel or culture for bacteria, a Shiga toxin test, C. difficile testing, and ova and parasite exams after travel.
• Blood tests: blood count, kidney function, electrolytes and inflammation markers.
• Fecal calprotectin: a stool marker of bowel inflammation, high in IBD.
• Flexible sigmoidoscopy or colonoscopy: for diarrhea lasting more than 1 to 2 weeks, suspected IBD, or older patients, with biopsies.
• CT scan: for severe pain or suspected ischemic colitis.

Treatment

Fluids first

Dehydration is the main short-term danger. Use oral rehydration solution (ORS) in small, frequent sips. Children should continue breastfeeding or normal feeds. Severe dehydration needs IV fluids.

Medicines

• Avoid loperamide and similar antidiarrheals with bloody diarrhea or fever unless a doctor advises it. They can keep toxin-producing bacteria in the gut longer and may worsen colitis.
• Antibiotics are given for Shigella, severe Campylobacter or Salmonella in high-risk people, amebic dysentery and C. difficile. They are not given for E. coli O157 because they may increase HUS risk.
• IBD treatment includes 5-ASA medicines, steroids, immunosuppressants and biologic drugs.
• Acetaminophen (paracetamol) for fever and pain. Avoid NSAIDs, which can worsen bleeding and kidney problems.

Prevent spreading it

Wash hands thoroughly after the toilet and before preparing food. Stay home from work, school or daycare until 48 hours after the last episode of diarrhea, and avoid swimming pools for 2 weeks. Food handlers and healthcare workers may need clearance tests.

Frequently asked questions

When should I worry about bloody diarrhea?

Always have bloody diarrhea checked by a doctor. Seek emergency care for heavy bleeding, dizziness, severe belly pain, high fever, signs of dehydration, or reduced urine in the week or two afterward. Children, older adults and people with weak immune systems need urgent assessment.

What is the most common cause of bloody diarrhea?

Bacterial infections are the most common cause of sudden bloody diarrhea, especially Campylobacter, Salmonella, Shigella and Shiga toxin-producing E. coli. When bloody diarrhea lasts more than a couple of weeks, inflammatory bowel disease such as ulcerative colitis is the most likely cause.

Can stress cause bloody diarrhea?

Stress can trigger diarrhea and flares of IBS, but IBS does not cause bleeding. Stress may worsen ulcerative colitis or Crohn's disease. Blood in diarrhea always has a physical cause that needs investigation.

What does blood and mucus in diarrhea mean?

Blood mixed with mucus suggests inflammation of the colon lining (colitis). Common causes are infections such as Shigella and Campylobacter, and ulcerative colitis. If it lasts more than a few days or comes back, your doctor will likely arrange stool tests and possibly a colonoscopy.

Should I take Imodium for bloody diarrhea?

No, not without medical advice. Loperamide (Imodium) slows the bowel and can keep harmful bacteria and their toxins inside longer, which may worsen colitis or raise the risk of complications such as hemolytic uremic syndrome after E. coli infection.