SYMPTOM
Blue or purple gums
The gums, tongue or lips take on a bluish, purplish or dusky-grey tinge instead of their usual pink, sometimes alongside laboured breathing, reluctance to move, or generally low energy.
Airway Obstruction
A narrowed or blocked airway, whether from brachycephalic anatomy, a collapsing trachea, a paralysed larynx, or an object lodged in the throat, reduces the volume of air reaching the lungs. Mucous membrane colour can shift within minutes of the obstruction worsening, particularly during exertion, heat or excitement.
Lung Or Pleural Space Disease
Pneumonia, fluid or air in the space around the lungs, or severe bronchial disease all interfere with the lungs' ability to move oxygen into the bloodstream, regardless of how much air is being breathed in. Because the underlying problem lies within the chest rather than the upper airway, breathing rate may already be increased even while air is still moving in and out relatively freely.
Heart Disease
Congestive heart failure, whether from mitral valve degeneration in small-breed dogs or cardiomyopathy in cats, can lead to fluid accumulating in or around the lungs. Some congenital heart defects also allow oxygen-poor blood to bypass the lungs entirely and reach the tongue and gums directly, producing a similar colour change from a different route.
Naturally Pigmented Mucosa
Certain breeds, Chow Chows and Shar Peis in particular, and occasionally Akitas, carry genetically blue-black pigmented tongues and patches of gum as a normal breed trait rather than a sign of poor oxygenation. This pigmentation is present from a young age and does not fluctuate with activity or breathing, which is worth distinguishing from a genuine, newly appearing colour change.
Reduced Oxygen-Carrying Capacity
Severe anaemia, or a toxin that alters haemoglobin's ability to carry oxygen, with paracetamol in cats being a well-recognised UK example, can produce a dusky or brownish-blue mucous membrane colour even when the lungs and heart are working normally. This category is worth considering particularly when there has been any possible access to human medication.
Why timing matters
Early observation
In the earliest stages, mild colour change often reflects a partially compensated drop in oxygen delivery, where the body is still managing to maintain function through increased breathing rate or effort. At this point the animal may still be alert and responsive despite the visible change.
Later presentation
As the underlying process continues, compensation fails and tissues throughout the body, not only the visible mucous membranes, become oxygen-starved. This is reflected in worsening weakness, disorientation or an inability to settle into a comfortable resting position.
Because cyanosis only becomes visible once a substantial proportion of circulating haemoglobin is desaturated, its appearance generally marks a more advanced stage of whatever cardiorespiratory or toxic process is underlying it. It is not, in that sense, an early warning sign but a comparatively late one.
Conditions commonly associated
Brachycephalic Obstructive Airway Syndrome (BOAS) in Dogs
Anatomical airway narrowing in flat-faced breeds can limit oxygen intake enough to produce visible mucous membrane colour change, especially with exertion or heat.
Laryngeal Paralysis
Failure of the laryngeal cartilages to open fully during inspiration restricts airflow and can produce cyanosis during exercise or excitement, most often in older large-breed dogs.
Pneumonia
Infection and inflammation within the lung tissue impairs oxygen transfer, and can present with laboured breathing and dusky mucous membranes.
Pneumothorax
Air trapped in the chest cavity outside the lung prevents normal lung expansion, reducing the volume of functional lung tissue available for oxygen exchange.
Congestive Heart Failure in Dogs
Fluid backing up into or around the lungs as the heart fails to pump effectively directly interferes with oxygen uptake, producing breathlessness and, in advanced cases, cyanosis.
Hypertrophic Cardiomyopathy in Cats
Thickened heart muscle can lead to fluid accumulating around the lungs or within the chest, a recognised cause of sudden breathing difficulty and mucous membrane colour change in cats.
Paracetamol Toxicity in Cats
Cats lack the liver enzyme needed to safely metabolise paracetamol, and even a small dose can convert haemoglobin into a form that cannot carry oxygen, producing a brownish-blue mucous membrane colour.
When to explore further
The colour change is accompanied by visibly increased effort to breathe, a wider stance with elbows held out, or open-mouth breathing in a cat
The colour worsens with activity and improves with rest, which points towards a cardiorespiratory limitation on oxygen supply
There is a known or possible exposure to paracetamol, onions, or another substance capable of affecting the blood, particularly in cats
The discolouration is new rather than having been present since a young age, distinguishing it from constitutional breed pigmentation
General collapse, extreme lethargy or reluctance to lie down comfortably accompanies the colour change
A photograph of the gums in natural light, taken squarely and without flash, provides a more reliable record than memory for comparing colour over time. Counting breaths per minute while the animal is resting quietly, and noting whether the colour and breathing pattern change with mild activity, helps characterise the pattern. A note of any access to medication packets, onions, garlic, or other household substances in the preceding hours is also useful context, particularly for cats.
Last reviewed: 12 September 2026 · Dr Alastair Greenway MRCVS