SYMPTOM
Bad breath
A persistent unpleasant odour from the mouth that has developed or worsened over time.
Dental and periodontal disease
The most common cause of persistent bad breath in companion animals is dental disease, including plaque and calculus accumulation, gingivitis, and periodontitis. As bacteria colonise the surfaces of teeth and extend beneath the gumline, they produce volatile sulphur compounds and other metabolic byproducts that create characteristic odours. Advanced periodontal disease with tooth root abscesses or bone loss tends to produce particularly strong, offensive breath.
Oral masses or lesions
Growths within the oral cavity — including benign and malignant tumours, polyps, or inflammatory lesions — can produce halitosis, particularly if the tissue becomes ulcerated, necrotic, or secondarily infected. The odour may have a distinctly different quality from dental disease, sometimes described as putrid or tissue-decay-like. These lesions may be visible on oral examination or may be located in areas that are difficult to inspect without sedation.
Kidney disease
When kidney function is significantly compromised, waste products that would normally be filtered and excreted accumulate in the bloodstream. Uraemia — the build-up of urea and other nitrogenous compounds — can produce a distinctive ammonia or urine-like odour on the breath. This uraemic breath is typically accompanied by other signs such as increased thirst, reduced appetite, and changes in urination patterns.
Diabetic ketoacidosis
In animals with uncontrolled diabetes, the body's shift to fat metabolism produces ketone bodies, which can give the breath a characteristic sweet, fruity, or nail-polish-remover-like odour. This acetone-like smell is typically associated with a more advanced metabolic derangement and is usually accompanied by other signs such as excessive thirst, increased urination, weight loss, and lethargy.
Gastrointestinal
Conditions affecting the oesophagus, stomach, or upper gastrointestinal tract can sometimes contribute to halitosis. Foreign bodies lodged in the mouth or oesophagus, gastric reflux, or inflammatory conditions of the upper digestive tract may produce breath changes. Some animals with coprophagia (consuming faeces) or who regularly eat strongly odoured items may also present with notable breath changes that reflect dietary rather than disease-related causes.
Why timing matters
Early observation
When bad breath is first noticed, it may reflect the early stages of dental disease, a dietary factor, or a transient oral issue. Mild halitosis in companion animals is relatively common and may not initially cause concern, but a change from the animal's baseline oral odour can be significant. Early-stage dental plaque accumulation, mild gingivitis, or retained food particles between teeth can produce a noticeable but often mild odour. At this stage, the breath change may be intermittent or only apparent at close range.
Later presentation
Bad breath that intensifies, develops a different character (such as a sweet, metallic, or ammonia-like quality), or persists despite dietary consistency may suggest a more established or systemic process. As dental disease progresses from gingivitis to periodontitis, the accumulation of bacteria beneath the gumline, tissue breakdown, and potential tooth root infection can produce increasingly pronounced and unpleasant odours. Halitosis with a non-dental character — particularly a fruity, chemical, or urine-like smell — may point toward metabolic conditions rather than oral pathology. Progressively worsening breath in older animals warrants particular attention.
The trajectory of halitosis depends heavily on its cause. Dental-related bad breath typically follows a gradually progressive course, worsening incrementally as plaque mineralises into calculus and gum disease advances. Metabolic causes may produce a more sudden shift in breath character that correlates with the onset or progression of the underlying condition. Some animals experience fluctuating breath quality influenced by diet, hydration, or periodic self-cleaning behaviours. In cats particularly, changes in breath odour may develop alongside subtle changes in eating behaviour, grooming patterns, or drooling that provide additional context.
Conditions commonly associated
Dental Disease in Dogs
Dental Disease in Cats
Stomatitis in Cats
The chronic inflammation, bacterial overgrowth, and tissue breakdown in stomatitis produce a characteristically strong, unpleasant breath odour that often worsens as the condition progresses.
Periodontal Disease
Halitosis is often the first owner-noticed sign, resulting from bacterial activity and tissue breakdown in the mouth.
Anal Gland Disease
Infected anal glands can contribute to foul breath when dogs lick the affected area and the distinctive odour transfers.
Feline Stomatitis
The severe oral inflammation and secondary bacterial involvement in stomatitis typically produces notably unpleasant breath.
When to explore further
Bad breath that has progressively worsened over weeks to months, particularly in a middle-aged or older animal, may indicate advancing dental disease or another process that could benefit from oral examination.
When halitosis is accompanied by changes in eating behaviour — such as reluctance to eat hard food, dropping food, chewing on one side, or pawing at the mouth — the combination suggests oral pain or discomfort alongside the breath change.
A sudden change in breath character, particularly to a sweet, fruity, ammonia-like, or metallic quality, may point toward a metabolic or systemic cause rather than a purely dental origin.
Bad breath accompanied by other systemic signs such as increased thirst, weight changes, reduced appetite, or lethargy may suggest that the halitosis is a manifestation of a broader health condition rather than an isolated oral problem.
Visible changes in the gums — redness, swelling, bleeding, recession, or the presence of lumps or masses — alongside bad breath may indicate that the oral tissues themselves are significantly affected.
Paying attention to the character and timing of the breath odour — whether it is consistently present or varies with meals, whether it has a particular quality beyond simply unpleasant, and whether it has changed from the animal's baseline — can help build a useful picture. Observing any concurrent changes in eating behaviour, drooling, pawing at the mouth, or interest in hard versus soft food may provide additional context about whether an oral source is likely.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS