Close-up of a dog's mouth - why does my dog's breath smell like fish
on August 31, 2026

Why Does My Dog's Breath Smell Like Fish? Causes & How to Fix It

If your dog's breath smells like fish, it usually comes down to one of three things: dental disease, anal gland issues, or diet — and the actual chemical behind that distinctive smell has a name: trimethylamine. This guide covers what's really going on, what different breath smells can tell you, the four real stages of dental disease behind most bad breath, how to get rid of it, and when it's worth a vet visit.

Written by Jamie Sinclair, who writes about canine health, nutrition, and everyday care for Doggy Dailies — focused on translating veterinary research and real ingredient science into practical, honest guidance UK dog owners can use.

Table of Contents

The Actual Chemistry Behind the Smell

That distinctive fishy odour isn't just a vague description — it has a specific chemical cause called trimethylamine (TMA), a compound produced during the digestion and breakdown of certain dietary compounds (choline, carnitine, and similar nutrients found in fish and other protein-rich foods). A 2022 study in Frontiers in Veterinary Science measured these exact compounds in dogs, confirming that diet directly affects circulating trimethylamine-related compounds — which is part of why a fish-based diet or fish oil supplement can genuinely change how a dog's breath smells.

Anal Gland Issues: The Most Surprising Cause

This one catches most owners off guard: dogs have two small anal glands that secrete a strong, musky fluid — and that fluid has a distinctly fishy smell. If these glands become impacted, infected, or overly full, a dog will often lick the area repeatedly to relieve the discomfort, which transfers the smell to their mouth.

Dogs also release the fluid when frightened, so a sudden fishy waft after a fright or a scuffle is normal. It's common: a UK study of over 104,000 dogs found anal sac problems in 4.4% in a single year, with spaniels and flat-faced breeds most affected (O'Neill et al., 2021, Veterinary Record).

If your dog's fishy breath is paired with scooting, excessive licking near their rear, or visible discomfort sitting, impacted anal glands are the likely culprit — your vet can check and express them, and our anal gland health guide covers prevention.

Dental Disease

Dental disease is the most common cause of bad breath in dogs generally — UK vet records show periodontal disease diagnosed in 12.5% of dogs in a single year, with small breeds at around three times the odds (O'Neill et al., 2021, JSAP). Other sources put the lifetime figure much higher still: the American Veterinary Dental Society estimates over 80% of dogs show some degree of periodontal disease by age three.

Plaque buildup allows bacteria to thrive along the gumline, leading to gingivitis and, eventually, periodontal disease if untreated. As bacteria break down food particles in the mouth, they release foul-smelling sulphur compounds that can sometimes take on a fishy quality rather than a purely "bad breath" smell.

Other signs of dental disease include visible tartar, red or bleeding gums, reluctance to chew, and pawing at the mouth. For a full breakdown of brushing technique, dental chews and the VOHC seal, see our complete guide to dog teeth cleaning.

The Four Stages of Dental Disease — and Which Ones Are Reversible

It's worth understanding the actual staging vets use, because it changes what "catching it early" really means. Dental disease in dogs is generally classified into four stages based on how much bone loss has occurred around the teeth:

Stage What's happening Reversible?
1 — Gingivitis Red, swollen gums and visible plaque or tartar; no bone loss yet Yes, with proper dental care
2 — Early periodontitis Mild bone and ligament loss begins; bad breath becomes more noticeable No, but manageable
3 — Moderate periodontitis Up to 50% bone support lost; teeth may loosen No, needs treatment
4 — Advanced periodontitis Over 50% bone loss, visible tartar, gum recession, likely extractions No, damage is permanent

The practical takeaway: Stage 1 is the only stage that's genuinely reversible with good dental care, which is exactly why catching bad breath early — rather than dismissing it as normal "dog breath" — is worth taking seriously. Everything past Stage 1 can be managed and treated by a vet, but the bone and ligament loss itself doesn't undo.

Diet and Digestion

Fish-based dog foods, fish oil supplements, and fish-based treats are a straightforward and common cause — the compounds involved pass through digestion and can be detected on the breath. Digestive issues more broadly (including reflux or a sensitive stomach) can also contribute to unusual breath odours, since what's happening in the gut often shows up in breath; if that's the pattern, our checklist on whether your dog needs a probiotic is worth a look.

What Different Breath Smells Can Tell You

The smell Most likely cause Urgency
Fishy Anal glands, dental disease, fish-based diet Low, unless scooting or gum signs
Rotten / sulphur Dental disease, a rotten tooth, something stuck in the mouth Book a dental check
Sweet or fruity Possible diabetes Vet soon, especially with thirst or weight loss
Urine / ammonia Possible kidney disease Vet soon
Faeces Usually scavenging (coprophagia); rarely a bowel problem Low, unless persistent

Other Possible Causes

  • Eating something unpleasant — dogs explore the world with their mouths, and scavenging (rubbish, faeces, or something found outdoors) is a common, usually harmless cause
  • A foreign object stuck in the mouth — worth checking if the smell appeared suddenly
  • Kidney or liver issues — less common, but persistent unusual breath alongside other symptoms (vomiting, appetite changes, lethargy, yellow gums) warrants a vet check

How to Get Rid of Fishy Dog Breath

Doggy Dailies Gland-EZe chews jar, natural anal gland support for dogs
  • Brush your dog's teeth — daily is the goal, with a dog-safe toothpaste (never human toothpaste); every couple of days still makes a real difference
  • Offer dental chews as part of their routine — useful as an extra, not a substitute for brushing
  • Sort the anal glands — if scooting or licking is part of the picture, firm stools help the glands empty naturally; our Gland-EZe chews combine pumpkin, prebiotic fibre and probiotics for exactly that, and our guide to the best diet for anal gland issues covers the food side
  • Check the diet — if it's fish-related, a short trial without fish-based treats or oils can confirm the cause; if you need the omega-3, a krill or algae source may smell less
  • Book a dental check if there's tartar or red gums — brushing won't shift established disease

A Realistic Brushing Routine

Most owners who try tooth brushing and give up do so because they start too fast. A more realistic approach: for the first few days, simply let your dog lick a small amount of dog-specific toothpaste off your finger so they associate the taste with something positive.

Once they're comfortable with that, introduce a finger brush or soft toothbrush without any scrubbing motion at first — just touching the teeth and gums briefly. Only once your dog tolerates that well should you move to genuine brushing, focusing on the outer surfaces of the back teeth, where plaque builds up fastest and where most owners never actually reach.

Short, calm, frequent sessions beat one long wrestling match, and daily is the ideal, but three to four times a week still meaningfully slows plaque buildup compared to not brushing at all.

When to See Your Vet

See your vet if the smell appears suddenly and doesn't improve, is accompanied by scooting or visible discomfort, or comes with other symptoms like vomiting, appetite loss, lethargy, increased thirst, or yellow gums. Persistent dental disease also needs proper veterinary attention rather than just brushing at home, since only Stage 1 gingivitis can be reversed without professional treatment.

FAQ

Why does my dog's breath smell like fish?
Most often it's dental disease, impacted anal glands, or a fish-based diet or supplement — the specific compound responsible is called trimethylamine.

My dog's breath smells like fish and they're scooting — what does that mean?
Almost certainly anal glands: the fluid smells fishy, and a dog licking an uncomfortable or full gland transfers the smell to their mouth. Ask your vet to check and express them, and look at firming up the stools.

Can anal glands really cause bad breath?
Yes — anal gland secretions have a distinctly fishy smell, and a dog licking an impacted gland can transfer that smell to their mouth.

Does fish oil make a dog's breath smell like fish?
Yes, it can — the same trimethylamine-related compounds found in fish are processed through digestion and can affect breath odour.

Is fishy breath in dogs a sign of a serious problem?
Not usually, but persistent or sudden fishy breath, especially with other symptoms, is worth a vet check to rule out dental disease, anal gland issues, or (rarely) kidney or liver problems. Sweet or urine-like breath is more concerning than fishy.

How can I get rid of my dog's fishy breath?
Regular tooth brushing, dental chews, checking anal gland health with your vet, firming up stools, and reviewing whether fish-based treats or supplements are the cause are the main starting points.

Can dental disease in dogs be reversed?
Only the earliest stage, gingivitis, is fully reversible with proper dental care. Later stages involve permanent bone and ligament loss and need ongoing veterinary management rather than reversal.

Sources

This article is for general information only and isn't a substitute for veterinary advice. If your dog's breath changes suddenly or is paired with other symptoms, please see your vet.

Related: More on Dog Health & Care