The Canine Ledger

Vomiting and Diarrhoea: When It Can Wait and When It Can't

Most stomach upsets in dogs settle on their own. Some are emergencies that look mild at first. The signs that mean phone now, today, or watch.

Written by thecanineledger

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Phone your vet or an emergency clinic now if your dog with vomiting or diarrhoea:

  • Is retching without bringing anything up, or has a swollen belly
  • Can’t keep water down, or keeps vomiting
  • Is passing a lot of blood, or stools that look like raspberry jam or black tar
  • Is weak, collapsed, or has pale gums
  • Is a puppy that isn’t fully vaccinated
  • May have swallowed a poison, a toy, a sock, a bone, string, or anything else that isn’t food

If you aren’t sure, phone. A vet would rather hear from you about a dog who turns out to be fine.

There is a sentence in the Merck Veterinary Manual’s chapter on sudden bloody diarrhoea that every owner should know. It says that “Marked, peracute fluid loss can result in hypovolemic shock before clinically recognizable dehydration” (Merck, Acute Hemorrhagic Diarrhea Syndrome in Dogs).

In plain terms, a dog can lose fluid so fast that they go into shock before they look dehydrated. Checking the skin on the back of the neck won’t always tell you in time.

Most vomiting and diarrhoea in dogs is not like that. A 2025 study of UK veterinary records estimated that about 1 in 12 dogs is seen for sudden diarrhoea each year, and more than 80% of them needed only one visit (O’Neill et al., PLOS ONE 2025). While that is the usual story, the hard part is that a serious case can look like a mild one for the first few hours.

Owners find this hard to judge, and that is no criticism. In a 2026 study, 1,772 UK owners were given short written descriptions of real cases. They rated the case as less urgent than a panel of vets did in 28.4% of responses, and a swallowed object stuck in the gut was among the conditions they were least likely to recognise (Farrow, O’Neill & Packer, PLOS ONE 2026).

There is no agreed veterinary checklist for owners deciding when to go. We looked for one and didn’t find one. This page builds one from the Merck Veterinary Manual’s chapters, and where it has to make a judgement, it leans towards phoning the vet.

1. Go now: the signs that can’t wait

Retching with nothing coming up, or a swollen belly. This can be bloat, where the stomach fills with gas and twists. It kills quickly, especially in large, deep-chested dogs. → Emergency Signs in Dogs: When to Go to the Vet Now

Vomiting that won’t stop. Merck’s guide for owners, describing a blocked gut, warns that “The dog may be unable to stop vomiting, which can lead to life-threatening consequences” (Merck, Disorders of the Stomach and Intestines in Dogs). A dog who brings up even small sips of water is in the same category.

A lot of blood. Merck describes the typical case of sudden bloody diarrhoea in small dogs as “An acute onset of profuse hemorrhagic diarrhea (often said to resemble raspberry jam) in a small- or toy-breed dog.” With treatment, most of these dogs recover. Merck puts deaths at “less than 10% in hospitalized dogs”, but says the mortality rate “can be high if untreated.”

Black, tarry stools. Merck notes that “Black, tarry feces may be a sign of bleeding in the stomach or small intestine.” This is digested blood, and it is easy to mistake for an ordinary dark stool.

Weakness, collapse or pale gums. These are signs of shock, whatever the cause.

An unvaccinated puppy. Parvovirus often starts quietly. Merck says “Initial clinical signs may be nonspecific (eg, lethargy, anorexia, fever) with progression to vomiting and hemorrhagic small bowel diarrhea within 24–48 hours” (Merck, Canine Parvovirus Infection). It also notes that “Approximately 25% of dogs may have nonhemorrhagic diarrhea.” No blood doesn’t rule parvo out.

Anything swallowed that isn’t food. This includes poisons, medicines, toys, socks, corn cobs and bones. Merck’s owner guide asks owners to tell the vet about “any access to string or yarn, fabric, sewing needles, or similar objects,” and about missing toys. → My Dog Ate Something: Why You Shouldn’t Make Them Vomit Without a Vet

Our note: A dog who can’t keep water down is on our “go now” list on our own judgement, from Merck’s line about a dog being “unable to stop vomiting.”

2. Phone today: get advice, even if they seem all right

These are the dogs who aren’t an emergency yet but shouldn’t be left to see how things go.

Vomiting more than once or twice in a day, or with other signs. Merck’s owner guide says that “vomiting that occurs more often than once or twice daily, and vomiting accompanied by blood, abdominal pain, depression, dehydration, weakness, fever, weight loss, or other signs requires a more detailed examination” (Merck, Vomiting in Dogs).

Belly pain. Merck describes a ““praying” position, with the hindquarters raised and chest and forelegs held close to the floor” as a sign of abdominal pain.

A dog who is very young, very old, small, or already ill. Merck notes that dogs “with kidney or heart disease may require intravenous fluids” while vomiting. Small and young dogs have less in reserve.

A dog who hasn’t improved after a day, or is getting worse. We haven’t found a source that gives an exact number of hours. This is a judgement, and your vet may give you a different one.

Our note: “Hasn’t improved after a day” is our threshold. Merck’s owner page treats vomiting of under three to four days with no other signs as short-term.

3. Watch at home: when it can usually wait

A dog who has been sick once or twice, or has a loose stool or two, and is otherwise bright, interested in their surroundings, and drinking, can usually be watched.

Merck’s owner guide is reassuring here: “Short-term or even occasional vomiting is generally not associated with other abnormalities.” It also notes that diarrhoea itself can be useful: it “can actually be a defense mechanism for the animal, helping it to eliminate harmful organisms and their toxins” (Merck, Introduction to Digestive Disorders of Dogs).

Watching means watching. Check on them through the day. Anything from §1 or §2 moves them up a level.

4. Food and water

Don’t take water away. Merck’s owner guide is firm: “water should never be withheld unless the dog is receiving supplemental fluids.” It explains that “Dehydration and other internal abnormalities are expected with vomiting, and withholding water can worsen these effects.”

Food is less settled. The same Merck page, written in 2018, describes withholding food for 24 hours for short-term vomiting, then offering small amounts of easily digested food. It ends with a caution: “Follow the treatment directions provided by your veterinarian carefully. Providing too much or too little water or food during this time can hurt your pet.”

We looked for trials of fasting or bland diets in dogs with ordinary stomach upsets and found none. The nearest is a small trial in puppies with severe parvovirus in hospital, a very different situation. Fifteen were fed by tube from 12 hours after admission, and fifteen got no food until their vomiting had stopped. The early-fed puppies’ appetite, vomiting and diarrhoea returned to normal a median of one day sooner, and they gained weight (Mohr et al., J Vet Intern Med 2003).

So we don’t give a feeding plan here. Phone your vet and ask what they want you to do about food. That is especially true for puppies, toy breeds and dogs with diabetes or another illness.

Our note: We give no home feeding plan. Merck’s 2018 owner page describes a 24-hour fast, but the only trial we found favoured early feeding in severe parvo. Ask your vet.

Our note: Singling out puppies, toy breeds and diabetic dogs is from general principles, not a source we quote.

5. Vomiting or bringing food back up?

It helps the vet to know which one you saw. Merck explains that vomiting is forceful and “is typically preceded by other signs, such as nausea, excessive drooling, retching, and forceful contractions of the abdominal muscles.”

Regurgitation is different. It “is a passive motion that does not require effort or contraction of the abdominal muscles,” and the food “tends to be undigested and may have a cylindrical shape reflecting the shape of the esophagus.” The two have different causes, so say which it looked like.

6. What to tell the vet

When you phone, have these ready:

  • When it started, and how many times your dog has vomited or had diarrhoea
  • What it looked like: blood, black stools, food, froth, anything they ate
  • Anything they could have eaten, including bins, medicines, plants, toys or socks that have gone missing
  • Whether they’re drinking, keeping water down, and peeing
  • Their age, vaccinations and any illnesses or medicines

Merck suggests asking “whether you should bring along a recent stool sample to help with diagnosis.” A photo of the vomit or stool can help too.

If the vet doesn’t prescribe antibiotics for mild diarrhoea, that is in line with the current evidence. → Does Diarrhoea Need Antibiotics? What the Evidence Says

Our note: Merck’s 2018 owner page and its 2025 professional chapter differ on antibiotics. We follow the newer chapter.

7. What we could not verify

Commonly saidWhat we found
“Check if the skin springs back. If it does, they’re not dehydrated”Not reliable in a fast-moving case. Merck notes shock can come “before clinically recognizable dehydration.”
“Starve them for 24 hours”Merck’s 2018 owner page describes this, but we found no trial in ordinary cases. One small trial in severe parvo favoured early feeding. Ask your vet.
“Chicken and rice settles a stomach”No trial found. We haven’t recommended any diet.
“No blood means it isn’t serious”Not true. Merck: about a quarter of dogs with parvo may have diarrhoea without blood.
An official list of red-flag signs for ownersNone found. The list on this page is built from Merck’s chapters.
An exact number of hours to wait before phoningNone found. “After a day” is our judgement.

Sources

Always confirm with your vet before acting on anything on this page.