Any blood in a dog's stool warrants a call to a vet. This page explains what the difference in appearance means, so that the call is more useful — not so that you can decide whether to make it.
Bright red blood
Fresh red blood, sometimes streaked on the surface of a formed stool and sometimes mixed through diarrhoea, has not been digested. That means it entered the gut close to the exit — the colon, the rectum or the anus.
Common reasons include colitis, straining, parasites, a dietary indiscretion, an anal gland problem, or a small injury from something sharp that was swallowed.
A single streak on an otherwise normal stool, in a bright dog that is eating and behaving normally, is generally a next-day appointment rather than an emergency. Repeated fresh blood, or blood with diarrhoea, moves it up the list.
Black, tarry stool
Black stool with a tarry consistency, sometimes described as looking like coffee grounds, is digested blood. It has travelled the length of the gut, which means the bleeding is higher up — the stomach or the small intestine.
This is generally the more serious of the two, and the causes include ulcers, which can be associated with anti-inflammatory medication, and bleeding disorders.
Black tarry stool warrants a same-day call, even in a dog that seems otherwise well.
The emergency picture
Some presentations need care within hours rather than days:
Large volumes of bloody diarrhoea, particularly if it is watery and has a distinctive smell, can indicate haemorrhagic gastroenteritis, and dogs can deteriorate quickly through fluid loss.
Pale, white or grey gums indicate significant blood loss and are an emergency regardless of anything else. Press a gum with a fingertip; the colour should return within about two seconds.
Blood alongside repeated vomiting, collapse, obvious pain, a swollen abdomen, or extreme lethargy is urgent. So is any blood in a puppy, a very small dog or an elderly dog, all of which have far less reserve.
Known access to a toxin — particularly rodenticides, which interfere with clotting — is an emergency even before any blood appears.
What to bring or send
Vets are entirely used to photographs, and a picture is more informative than a description. If you can collect a sample in a clean bag or container, that is better still, because it allows testing for parasites without waiting for another opportunity.
Be ready to say how long it has been going on, how many times, whether the dog is eating and drinking, what medications it is on — NSAIDs and steroids are particularly relevant here — and whether it could have eaten anything unusual, including bones, toys, rubbish or anything from a bin.
What not to do
Do not give human anti-diarrhoeal medication. Several are unsafe in dogs, and slowing the gut down can be actively harmful when the cause is an infection or a toxin.
Do not withhold water. Withholding food briefly is sometimes appropriate on veterinary advice; withholding water is not, and dehydration is the thing most likely to turn a manageable problem into a serious one.
This page is background for a conversation with a vet, not a substitute for one.
Why the same appearance can have very different urgency
Two dogs can present with identical stools and need completely different responses, and what separates them is everything else about the dog.
A middle-aged dog in good condition, eating normally, bright and playful, with one streak of fresh blood after a bout of straining, is a very different situation from a dog with the same streak that is also quiet, off its food and reluctant to move. The blood is the same; the context is not.
That is why the questions a vet asks over the phone are mostly not about the stool. They are about appetite, energy, drinking, vomiting, gum colour and pain — because those determine whether this is something to see today or something to see this week.
Recording it properly
If the problem recurs over days, a short written record is genuinely useful: date and time, appearance, whether the dog strained, what it had eaten, and how it seemed otherwise. Patterns emerge in writing that are invisible in memory, and a dog that has blood only after a particular treat, or only on the days it is walked somewhere specific, has told you something valuable.
Bring that record and a photograph. Between them they usually answer most of the history-taking before it starts.