Last updated October 22, 2026
A fishy smell on a dog's breath usually comes from one of three places. The anal sacs under the tail hold a strong fish-smelling fluid, and a dog that licks its rear end carries that smell to its mouth. Fish in the food, treats or a fish oil supplement can do the same after meals. And dental disease, the most common cause of bad breath in dogs, can leave a fishy note too, through the gases that plaque bacteria on the teeth give off.
A short sniff test, set out below, tells you which one you're dealing with. If the smell is sudden, keeps coming back or comes with scooting, red gums, drooling or vomiting, see your vet. Breath that smells of ammonia, sweet or like poop points elsewhere, and our guide to what other breath smells mean covers those. For the gum and tooth problems behind most mouth odor, see our overview of dental disease in dogs.
Where a fishy smell comes from
The molecule behind the smell of rotting fish is trimethylamine. When Preti and colleagues (1976) analyzed the fluid from dogs' anal sacs, trimethylamine was one of its major parts. So a sharp, clean fish smell is the scent you'd expect from that fluid.
Bad breath from the mouth is mostly different chemistry. Bacteria in plaque, in the pockets around the teeth and on the tongue make hydrogen sulfide and methyl mercaptan (Culham and Rawlings, 1998). US health agencies describe the first as smelling of rotten eggs, and the second of garlic or rotten cabbage. Mixed together in a dog's breath, they can still come across as fishy, because noses differ.
That's why we don't sort the cause by smell alone. A dog can have plaque and full anal sacs at the same time, so the test below checks each place in turn.
A sniff test to find the source
Do all three checks, in this order, on the same morning.
- Before breakfast, lift your dog's upper lip at the side and wipe a clean gauze pad or cotton pad along the gums of the big back teeth. Wait a few seconds and sniff the pad. Rawlings and Culham (1998) found that dogs producing a lot of breath gases smelled worst early in the morning, before feeding, so this is when a mouth smell is easiest to catch.
- Sniff the fur under the tail and the bed or blanket your dog sleeps on. Over the next day or two, watch for scooting and for licking or nibbling at the rear end.
- Read the ingredient list on the food and the treats, and smell the breath again soon after a meal.
| What you find | Most likely source | Next step |
|---|---|---|
| The gauze pad smells fishy, rotten or sour | The mouth | Dental check with your vet, then daily home care |
| Fishy fur or bedding, scooting, licking the rear | The anal sacs | Vet checks and empties them |
| Fish, fish meal or fish oil on the label, smell strongest after meals | The food | Nothing to fix |
| None of these, or the smell comes with vomiting, diarrhea or a dog that seems unwell | Something else | Vet, the same day if your dog is weak or vomiting |
Anal sacs
Dogs have two small anal sacs, one on each side of the anus. The Merck Veterinary Manual describes them as related to the scent glands in skunks, making a small amount of foul-smelling fluid that is normally squeezed out when a dog passes stool. When the sacs clog or get infected, dogs scoot, lick or bite at the area, and may strain to go.
That's how the smell reaches the breath. A dog that keeps licking an itchy rear end ends up with the fluid on its tongue.
It's a common problem. In a study of 104,212 dogs seen at UK vet practices, 4.4% had an anal sac disorder diagnosed within one year (O'Neill and colleagues, 2021). The rate was far higher in some breeds, about 1 in 7 Cavalier King Charles Spaniels and around 1 in 12 Cockapoos, and flat-faced breeds had 2.6 times the odds of long-nosed breeds. If you own one of those breeds, check the rear end before you blame the teeth.
Leave the emptying to your vet, who can also tell an impacted sac from an infected one. Merck lists added fiber in the diet among the treatments, so ask your vet before changing food. A swelling beside the anus, a red or purple patch, blood or pus, or a dog in pain needs a prompt visit, since an infected sac can form an abscess.
Fish in the food bowl
Fish isn't always in the name on the front of the bag. Check the ingredient list for salmon, white fish, fish meal, fish oil or salmon oil, which can turn up in recipes named after chicken or lamb. Fish treats and fish skins count too.
Fish oil supplements can do it as well. In a 90-day trial of fish oil capsules in 18 Miniature Schnauzers, the only side effect owners reported was a strong fish odor (de Albuquerque and colleagues, 2021). The paper doesn't say whether the smell was on the breath or the coat. If your vet prescribed fish oil for your dog's skin, joints or blood fats, don't stop it because of the smell. Ask your vet whether a different form or dose would suit.
A food smell is strongest right after eating and fades over the next hours. If that's the pattern and the gauze pad from the back teeth smells clean, there's nothing to treat.
When the mouth is the source
If the gauze pad smells, the usual cause is plaque and the gum inflammation it brings. In a small 1997 study of beagles, the strength of the breath smell rose with gum inflammation and with bone loss between the roots (Simone and colleagues). Our guide to the stages of periodontal disease explains what each stage means for your dog.
Start with a dental check. Brown or yellow tartar that won't wipe off has to come off under anesthesia, and the American Veterinary Dental College says the part below the gumline can't be cleaned on an awake dog. After that, daily brushing is the home care with the best evidence, and our guide to cleaning your dog's teeth at home shows how to start.
If brushing is a fight, a daily supplement sprinkled on food is another option. Caniclean is our seaweed supplement for dogs' teeth, and it helps reduce plaque and new tartar. It won't lift tartar that has already hardened, and it isn't VOHC-accepted. Seaweed contains iodine, so ask your vet first if your dog has a thyroid condition, takes thyroid medication or has kidney disease. One owner of a 15-year-old dog wrote in a review on May 27, 2024, "he had a lot of tartar and his breath was awful! He's not a fan of brushing, so Caniclean has been an amazing find."
When to see a vet
- A fishy smell that is new and sudden, or still there after two or three weeks of daily care
- Scooting or licking the rear end, especially with swelling, redness, blood or pain
- Red, swollen or bleeding gums, a loose or broken tooth, drooling or dropping food
- Something stuck between the teeth or in the roof of the mouth
- Vomiting, diarrhea or burping along with the smell
- Breath that smells of ammonia or urine, or sweet and fruity, which can point to kidney disease or diabetes
Go the same day if your dog is weak, vomiting repeatedly or won't eat.
Questions owners ask
Why does my dog's breath suddenly smell like fish?
A sudden fishy smell often comes from full or irritated anal sacs, especially if your dog has started scooting or licking its rear. A new food, fish treat or fish oil supplement can do it too. If neither fits, a vet should look in the mouth for an infected tooth or something stuck.
Is it normal for a dog's breath to smell like fish after eating?
Yes, if the food or treats contain fish or fish oil and the smell fades within hours. A smell that is there before breakfast too points to the mouth or the anal sacs.
Should I express my dog's anal glands at home?
We'd leave it to your vet or a veterinary technician. Squeezing too hard can hurt, and an impacted or infected sac needs a vet's treatment, not just emptying.
Can fish oil give my dog fishy breath?
It can leave a fishy smell. Owners in a 90-day fish oil trial noticed a strong fish odor, though the paper doesn't say if it was on the breath. Don't stop a supplement your vet prescribed. Ask about a different form or dose instead.
References
- American Veterinary Dental College. Companion animal dental scaling without anesthesia (position statement). avdc.org.
- Centers for Disease Control and Prevention, ATSDR. Medical management guidelines for hydrogen sulfide. cdc.gov.
- Centers for Disease Control and Prevention, NIOSH. Pocket guide to chemical hazards, methyl mercaptan. cdc.gov.
- Culham N, Rawlings JM. Oral malodor and its relevance to periodontal disease in the dog. J Vet Dent. 1998;15(4), 165-168.
- de Albuquerque P, De Marco V, Vendramini THA, et al. Supplementation of omega-3 and dietary factors can influence the cholesterolemia and triglyceridemia in hyperlipidemic Schnauzer dogs, a preliminary report. PLoS One. 2021;16(10), e0258058.
- MedlinePlus Genetics, US National Library of Medicine. Trimethylaminuria. medlineplus.gov.
- Merck Veterinary Manual. Disorders of the rectum and anus in dogs (updated September 2024). merckvetmanual.com.
- O'Neill DG, Hendricks A, Phillips JA, Brodbelt DC, Church DB, Loeffler A. Non-neoplastic anal sac disorders in UK dogs, epidemiology and management aspects of a research-neglected syndrome. Vet Rec. 2021;189(2), e203.
- Preti G, Muetterties EL, Furman JM, Kennelly JJ, Johns BE. Volatile constituents of dog (Canis familiaris) and coyote (Canis latrans) anal sacs. J Chem Ecol. 1976;2(2), 177-186.
- Rawlings JM, Culham N. Studies of oral malodor in the dog. J Vet Dent. 1998;15(4), 169-173.
- Simone AJ, Logan EI, Livgren R, Suelzer M. Oral malodor in beagles, association with indicators of periodontal disease. J Clin Dent. 1997;8(6), 163-168.