Why do cats hairball
Cats vomit hairballs because their grooming coats ingest fur that clumps in the stomach; this article explains the mechanism, risks, and when to see a vet.
Cats vomit hairballs because their grooming coats ingest fur that clumps in the stomach into a mass called a trichobezoar, which they cannot pass through the intestines and must regurgitate. This is a normal physiological response for most cats, though the frequency depends on how much they groom and how their digestive system handles the fiber.
The grooming trap
Cats are obligate carnivores with a digestive system designed for meat, yet they spend a significant portion of their day grooming. Their tongues are covered in backward-facing keratin spines called papillae, which act like a comb to remove loose fur, dirt, and parasites. Every lick deposits hair into the mouth, which is then swallowed. Unlike humans, who can digest hair to a small degree, the feline stomach cannot break down the keratin in fur. Instead, the hair accumulates, mixing with stomach acid and food particles to form a tight, elongated cylinder of packed fur.
Why they cannot pass it
The feline gastrointestinal tract is not built to move large, dry masses of hair through the intestines. While small amounts of hair may pass in the feces, a larger accumulation forms a trichobezoar. The stomach lining does not provide enough friction to push this mass further down the digestive tract. When the mass reaches a critical size, the cat's body triggers a vomiting reflex to expel it. This is distinct from vomiting caused by illness; it is a mechanical clearance mechanism for a foreign object the body cannot digest.
When it becomes a problem
Occasional hairballs are normal, but persistent vomiting warrants clinical evaluation. If a cat is vomiting frequently, losing weight, or showing signs of lethargy, the issue may not be simple hair accumulation. Hairballs can be mistaken for other conditions such as lymphosarcoma, tuberculosis, or tumors of the spleen. In severe cases, a trichobezoar can become a fatal obstruction if undetected, requiring surgical intervention. Rabbits face a similar risk but cannot regurgitate, making hairballs immediately dangerous for them, whereas cats can usually clear the blockage themselves.
Can cat grass help?
Many owners use cat grass, which consists of young cereal grasses like wheat, barley, oat, or rye, to aid in passing hairballs. The fiber content in these grasses is believed to help move hair through the digestive tract or stimulate the vomiting reflex to expel the mass before it becomes too large. A 2025 microscopy study of regurgitated matter found leaf trichomes interlocked with hair, suggesting a physical interaction. However, surveys of pet owners have not consistently observed behavioral changes to support the hypothesis that grass significantly reduces hairball frequency in all cats.
When to see a vet
While a single hairball is a minor inconvenience, frequent vomiting is a sign of an underlying issue. If a cat is vomiting hairballs more than once a month, or if the vomit contains blood, bile, or undigested food, a veterinary visit is necessary. Sudden changes in appetite, weight loss, or signs of pain should also trigger a professional assessment to rule out gastrointestinal disease. The goal is to distinguish between a normal grooming byproduct and a sign of a more serious health condition.
Understanding the mechanics of hairball formation helps owners distinguish between normal grooming behavior and potential health risks. While cat grass and grooming aids can assist, the most effective strategy is recognizing when the body's natural clearance mechanism is failing and seeking professional advice.