A rotten egg smell coming from your cat is almost always caused by sulfur compounds, whether from their digestive system, anal glands, or occasionally a sign of an underlying health issue. The smell can range from a brief, passing whiff to a persistent odor that follows your cat around the house. Here’s what’s behind it and what each source of the smell looks like.
Anal Glands Are the Most Common Culprit
Cats have two small sacs on either side of their anus that produce a potent, oily secretion. This fluid contains fatty acids, aldehydes, alcohols, and ketones, all of which combine to create a smell that many owners describe as sulfurous or egg-like. Normally, a tiny amount of this fluid is expressed naturally each time your cat has a bowel movement. You’d rarely notice it.
The problem starts when those sacs don’t empty properly. Impacted anal glands hold onto that concentrated fluid, and the smell becomes much more noticeable, sometimes releasing in small amounts onto furniture, bedding, or your lap. Cats with impacted glands often scoot their rear along the floor, lick or bite at the area excessively, or strain during bowel movements. If the glands become infected or abscessed, you may notice swelling, redness, or discoloration near the anus, along with obvious pain when your cat sits down.
Some cats express their anal glands suddenly when they’re startled or stressed. If you’ve noticed the rotten egg smell only during car rides, vet visits, or moments of fear, a stress-related gland release is the likely explanation. This is normal and not a sign of disease.
Gas and Diet
If the smell seems to come and go, especially after meals, flatulence is a strong possibility. The rotten egg odor in cat gas comes from the same compound responsible for the smell in human gas: hydrogen sulfide. Bacteria in your cat’s gut break down sulfur-containing compounds in food, producing hydrogen sulfide, methanethiol, and dimethylsulfide as byproducts. These are present only in trace amounts, but they’re potent enough to fill a room.
High-protein diets are a common trigger, since protein-rich ingredients contain sulfur-containing amino acids that gut bacteria readily ferment. Sudden food changes can also spike gas production as the bacterial population in your cat’s intestines adjusts. If your cat got into table scraps containing onions, broccoli, cabbage, or other high-sulfate foods, that can also dramatically increase the sulfur smell. Even certain commercial cat foods with carrageenan (a common thickener in wet food) may contribute.
Occasional gas is normal. But if your cat is consistently producing foul-smelling gas, especially alongside soft stools, vomiting, or weight loss, a digestive issue may be involved.
Digestive Conditions That Increase Sulfur Production
Persistent, unusually smelly gas or stool can point to a problem with how your cat absorbs nutrients. When food isn’t properly broken down and absorbed in the small intestine, it passes further along the digestive tract where bacteria ferment it, producing excess sulfur gases. This is the basic mechanism behind malabsorption syndromes.
One common version of this is small intestinal bacterial overgrowth, where the bacterial population in the small intestine grows beyond normal levels. These bacteria compete for nutrients, break down bile salts in abnormal ways, and can interfere with your cat’s ability to absorb certain vitamins, particularly B12. Cats with this kind of imbalance often have chronic diarrhea, poor coat quality, and gradual weight loss alongside the smell.
Food intolerances and inflammatory bowel disease can produce similar patterns. If the rotten egg smell from your cat has been getting worse over weeks or months rather than appearing suddenly, a chronic digestive condition is worth investigating.
Breath That Smells Like Sulfur or Worse
If the smell is specifically coming from your cat’s mouth, the possibilities shift. Dental disease is the most frequent cause of foul cat breath. Tartar buildup, gum infections, and tooth decay all create environments where odor-producing bacteria thrive. The smell from dental problems tends to be more generally “rotten” than specifically egg-like, but sulfur compounds are part of the mix.
A more concerning possibility is kidney disease. When the kidneys can’t filter waste efficiently, metabolic byproducts build up in the bloodstream and can give your cat’s breath a urine-like or ammonia-like odor. Some owners describe it as a chemical or sulfurous smell. Cornell’s Feline Health Center notes that this breath odor, combined with excessive thirst, increased urination, and loss of appetite, is a recognizable pattern of kidney problems. Kidney disease is especially common in older cats and progresses gradually, so the breath change may be subtle at first.
How to Narrow Down the Source
Figuring out where the smell originates helps you decide what to do next:
- Rear end, sudden burst: Anal gland expression, likely from stress or a full gland. If it happens rarely, it’s normal. If your cat is scooting or licking the area, the glands may be impacted.
- Rear end, after meals or ongoing: Flatulence from diet or a digestive issue. Try noting whether it correlates with specific foods.
- Mouth: Dental disease or a metabolic condition. Lift your cat’s lip and look for red gums, brown tartar, or broken teeth.
- Whole body or fur: Your cat may have sat in something, or anal gland fluid may have leaked onto their coat. A bath may solve it, but recurring episodes suggest a gland problem.
Reducing the Smell
For diet-related gas, switching to a food with a different protein source or fewer sulfur-heavy additives often helps. Transition gradually over a week to avoid making the gas worse during the switch. Look at the ingredient list on wet foods for carrageenan, and consider trying a brand without it.
For anal gland issues, increasing dietary fiber can help your cat express the glands naturally by producing firmer, bulkier stools. Pumpkin (plain, not pie filling) is a commonly used fiber source for cats, and commercial fiber supplements designed for anal gland health use blends of soluble and insoluble fiber like pumpkin seed and apple pectin. If the glands are already impacted or infected, a vet will need to manually express or treat them before dietary changes can prevent recurrence.
If the smell is coming from your cat’s breath and doesn’t resolve within a day or two, especially in a cat over seven years old, bloodwork can check kidney function and rule out metabolic causes before they progress.