Bloody Diarrhea: What Causes It and When to Go to the Emergency Room

Clinically reviewed by Dr. Ponlawat Pitsuwan, Physician, Doctor Samui. Last reviewed: September 2026

You felt fine at lunch. Maybe it was the grilled chicken at a beach shack near Chaweng, the papaya salad from the night market, or the prawns at a resort restaurant along Lamai. By evening you are in the bathroom every 30 minutes. Then you look down and see blood. That is the moment this stops being a bad stomach and becomes something that needs a doctor.

Bloody diarrhea is a different clinical situation from ordinary loose stools. Bright red blood in the toilet, dark tarry stool, or anything that looks like coffee grounds all mean something specific, and the colour matters for how urgently you need care. This article tells you what causes bloody diarrhea in Koh Samui, what the blood colour tells you, what not to do before you are seen, and when to stop waiting and get help.

What the Colour of the Blood Actually Tells You

This question comes up constantly, and it is one of the most useful things to know before you do anything else.

Bright red blood mixed in with your stool, or coating it, usually means the bleeding is in the lower gut: the colon or rectum. In travellers on Koh Samui, this is most often a bacterial infection causing inflammation in the gut wall. It is urgent, but it is usually not the most dangerous category.

Dark, tarry, black stool is a different matter. That colour means blood has been digested, so the source is higher up: the stomach or small intestine. Black tarry stool needs emergency care, not a wait-and-see approach.

Coffee-ground appearance in your stool or vomit also signals digested blood from the upper gut. Treat it the same as black tarry stool. Go to the ER or call a doctor immediately. Do not wait for a second episode.

The Bacteria Most Likely Behind Bloody Diarrhea in Thailand

Most cases of bloody diarrhea in travellers on Koh Samui come from one of four bacterial or parasitic infections, not a generic stomach bug.

Campylobacter jejuni is the most common bacterial cause in travellers across Southeast Asia. It comes from undercooked chicken and seafood, and cramping with blood in stool usually starts within two to five days of eating the contaminated food. Shigella causes classical dysentery, spreads through contaminated water and food, and can produce high fever with very frequent bloody stools.

Vibrio bacteria live in warm coastal water and undercooked seafood. If you have had oysters, raw clams, or reef-adjacent seafood in the days before your symptoms started, Vibrio is worth knowing about. Entamoeba histolytica causes amoebic dysentery, a parasitic infection from contaminated water or unwashed produce that can drag on for weeks without treatment.

Each of these needs a different antibiotic or antiparasitic drug. Without a stool test, treatment is guesswork.

Why Bloody Diarrhea Hits Harder on a Tropical Island

Koh Samui’s heat and humidity speed up fluid loss in ways most visitors do not expect. When you have bloody diarrhea, you are losing not just water but electrolytes: sodium, potassium, and chloride. In 32-degree heat, that imbalance can become serious dehydration far faster than it would at home.

The warning signs are dizziness when you stand up, a dry mouth, dark urine or no urine for several hours, and feeling weak or confused. If those signs appear alongside bloody diarrhea, oral rehydration salts alone are not enough. You need IV fluids, and you need them soon.

This is the situation where a hotel doctor visit makes practical sense. When you are too weak to sit in a waiting room, Doctor Samui can bring IV rehydration to your room.

Do Not Take Imodium or Cipro Before You See a Doctor

This is where beach wisdom gets it badly wrong, and acting on it can make things worse.

Loperamide (Imodium) stops your gut from moving. With ordinary loose stools, that can help. With bloody diarrhea, it traps bacteria and toxins inside your gut, and with certain pathogens it can contribute to a serious complication called toxic megacolon. Do not take it if there is blood in your stool.

Ciprofloxacin (Cipro) is the antibiotic many travellers carry from home. The problem is that Campylobacter jejuni, the most common bacterial cause of bloody diarrhea in this region, is resistant to fluoroquinolones like Cipro in more than half of cases across Southeast Asia. Taking it may do nothing, and it delays effective treatment. The preferred first-line drug here is azithromycin, but a stool test should guide which drug you actually need. For more detail on when diarrhea becomes an emergency in Koh Samui, that page walks through the full picture.

When to Go to the Emergency Room Right Now

Some symptoms mean stop waiting and go immediately, or call for help to come to you.

Go now if you have black or tarry stool, or coffee-ground vomiting. Go now if you have a fever above 39°C that is not coming down. Go now if you feel dizzy when you stand, or have not urinated in six or more hours. Go now if you are passing mostly blood rather than stool. Go now if you feel confused, very weak, or your heart is racing.

For everything short of those signs, a same-day doctor visit is still the right call. Bloody diarrhea with a fever above 38.5°C, bloody diarrhea that is not improving after 24 hours, or bloody diarrhea in a child of any age all need clinical assessment, a stool test, and a treatment plan.

Bloody Diarrhea in Children on Holiday

Children dehydrate faster than adults. What takes two days to become serious in an adult can happen in hours in a young child.

If your child has blood in their stool and a fever above 38.5°C, that is same-day medical care with no exceptions. Watch for sunken eyes, a dry mouth, fewer wet nappies than usual, or unusual drowsiness. In infants, a sunken soft spot on the top of the head is a sign of significant dehydration.

Antibiotic choices for children differ from adults. Fluoroquinolones like Cipro are generally not used in young children. Azithromycin is preferred for most bacterial causes in paediatric cases, and a doctor needs to choose the right drug and dose. Doctor Samui sees children regularly and can visit your hotel room so you do not have to move a sick child across the island.

A rare but serious complication called hemolytic uremic syndrome can follow certain bacterial gut infections in children and affects the kidneys. If your child develops reduced urination, pallor, or swelling after bloody diarrhea, get emergency care immediately.

What a Doctor Will Actually Do

When you are assessed for bloody diarrhea, the first step is checking how dehydrated you are. If you need IV fluids, that starts before anything else.

A stool sample will be sent for testing to identify the specific pathogen. Blood tests check your inflammatory markers and kidney function. Treatment usually starts the same day, most likely with azithromycin while the stool test results come back.

Most people with bacterial bloody diarrhea feel significantly better within 48 to 72 hours of starting the right antibiotic. If the stool test identifies Entamoeba histolytica, a different drug called metronidazole will be used instead.