Horse Side Vet Guide ®

Equine Health Resource

Membranes of Mouth, Gums appear Brown

Code Red - Call Your Vet Immediately, Even Outside Business Hours

Code Orange - Call Your Vet at Their First Available Office Hours

Code Red - Call Your Vet Immediately, Even Outside Business Hours

  • If the results of the Whole Horse Exam (WHE) in the resting horse indicate fever (Temp>101F/38.3C), or heart rate greater than 48 BPM.
  • If the horse seems to be breathing rapidly despite not having exercised.

Code Orange - Call Your Vet at Their First Available Office Hours

  • If the horse's appetite and attitude are normal and you see nothing else wrong.

There are a few areas on the body that provide important information regarding the state of a horse’s circulatory health. These areas include the gums, the pink membrane inside the nostril, the whites of the eyes, and the pink inner vulvar membranes of mares. That said, the simple act of evaluating the color of your horse’s gums is actually a very important skill.

True brown (as opposed to red, blue, or purple) mucous membranes are a very rare observation that classically indicates methemoglobinemia, a problem with hemoglobin (the oxygen carrying molecule in red blood cells) caused by nitrate toxicity (usually in the form of fertilizer but sometimes concentrated in certain green plants).

A horse with truly brown mucous membranes likely exhibits other signs of serious illness like high heart and respiratory rates, depression and loss of appetite. However, horses that graze certain kinds of grasses can also have dark stained (brown to black) gums.


Reexamine your horse’s gums in different light. Always check both sides of the mouth if you think your horse’s gums are a strange color. Wipe the gums clean with a moist paper towel. There may be material on the gums that is confusing your interpretation. Keep in mind that dull, toxic or dehydrated mucous membranes more commonly have a reddish, blueish or purplish appearance.

When in doubt, assess your horse’s general health using the Whole Horse Exam (WHE), paying particular attention to capillary refill rate and heart rate. Look for other signs of illness or disease. Especially check the color of the whites of the eyes, and if the horse is a mare, check the appearance of the vulvar membranes for comparison. If your horse was possibly exposed to nitrates or other toxins, share any pertinent information on the product label with your vet.

Contact your vet with your findings and concerns.


Your vet may advise you to take a “wait and see” approach or suggest that they examine your horse. Much of this will depend on history and the presence or absence of other concerning signs.

If an exam is recommended, your vet will assess your horse’s general health to try to determine the nature of the finding. Blood work and additional diagnostics may provide additional important information.

Author: Doug Thal DVM Dipl. ABVP


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