Why do dogs and cats eat grass? The vomiting myth debunked

August 09 2026, by Michael van Wassem, 6 min reading time

Does your dog or cat sometimes eat grass? Scientific research debunks the popular "they want to vomit" myth, and there is a risk that is often overlooked.

Your dog dives straight into a patch of grass during the morning walk, or your cat nibbles at the greenery on the windowsill. Sometimes a burp follows, or a small puddle of vomit on the carpet. Sound familiar? The explanation most people know is that animals eat grass "to make themselves sick". Scientific research shows that this idea is not correct.

Short answer: why do dogs and cats eat grass?

Eating grass is completely normal behaviour in both species and is usually not a sign of illness. Research from the University of California–Davis showed that 79% of dogs with daily access to plants actually ate them, and that only 9% of these dogs showed signs of being unwell before grazing. In cats, instinct, a need for fibre, and help with clearing hairballs likely all play a role. For both species, occasional grass eating is part of being a healthy, curious animal.

Why exactly do they do this?

In dogs

Dogs are descended from wolves, and after making a kill, wolves also eat the often partly digested plant matter found in their prey's stomach. This habit seems to have partly carried over. Researchers also point to three further explanations: grass provides roughage and fibre that support digestion, the texture and taste simply appeal to some dogs, and boredom or too little mental stimulation can reinforce the behaviour.

In cats

Cats are strict carnivores, but in the wild they often gnaw on the stomach contents of their prey after a hunt, including plant material. Grass also contains folic acid and trace elements, and its fibre helps move swallowed hair through the body, either by vomiting or in the stool. Cats that groom themselves a lot therefore tend to graze slightly more often than other cats.

The persistent myth: "they eat grass to make themselves sick"

💡 In a widely cited study by Sueda, Hart and Cliff (2008, published in Applied Animal Behaviour Science), only 22% of the dogs studied vomited regularly after eating grass, and fewer than 10% showed any signs of feeling unwell beforehand. Grazing therefore proved to be largely unrelated to illness or nausea: for most dogs, it is simply part of normal foraging behaviour.

This does not mean that vomiting after eating grass never happens. The sharp, hard-to-digest blades can mechanically irritate the stomach lining, which can still make some animals retch. This is a side effect, not the purpose of the behaviour.

When should you actually be concerned?

Occasional grazing with no other symptoms is no cause for concern. Do watch out for the following signs:

• A sudden, sharp increase in the amount of grass eaten
• Repeated vomiting, lethargy, or reduced appetite around the time of grazing
• Diarrhoea, or blood in vomit or stool
• Eating non-food items too, such as soil, stones, or dust (this is called pica and can point to a deficiency, a digestive issue, or stress)
• Unexplained weight loss

If you notice one or more of these signs, the right next step is a visit to the vet. The cause could be a food intolerance, parasites, a gastrointestinal condition, or a dietary deficiency.

A risk that is often overlooked: lungworm via slug slime

Something few articles on this topic mention: grass can be a route of infection for lungworm (Angiostrongylus vasorum, also known as French heartworm). Snails and slugs carry the larvae of this worm. A dog that eats grass covered in slug slime, or that accidentally swallows a slug, can become infected. Lungworm occurs across Europe, including in the Netherlands, and an untreated infection can lead to serious heart and lung problems.

⚠️ Do not let your dog drink from outdoor water bowls or play with toys that have been left outside without checking them first, and ask your vet whether your worming schedule also protects against lungworm. Not every standard product covers this.

Is it a nuisance? Here's a practical approach

Would you rather reduce the grazing, for example because your dog vomits afterwards or because your garden has been treated with chemicals? This approach helps:

• Check the diet. A food with enough fibre often reduces the need to look for roughage elsewhere. For cats with sensitive digestion, a specially formulated cat food for sensitive digestion can help.
• Support gut flora. For a dog that grazes more often after loose stools or stress, a probiotic supplement can help restore the balance. If in doubt, discuss this with your vet.
• Reduce boredom. Grazing out of boredom often decreases once a dog is mentally challenged, for example with a puzzle toy such as the KONG Treat Spiral Stick.
• Keep worming treatments up to date. An up-to-date deworming treatment limits the risk of grass-related parasites becoming a problem.
• Avoid treated grass. Stay away from lawns that have recently been treated with weedkiller or fertiliser, and check whether there are any poisonous ornamental plants in your own garden.

Checklist: does it need action or not?

• Eats a bit of grass now and then, otherwise fit and active → no action needed
• Occasionally vomits after grazing, otherwise healthy → keep an eye on it, no immediate concern
• Regular vomiting, diarrhoea, or lethargy → book an appointment with the vet
• Also eats soil, stones, or other non-food items → ask the vet about this (pica)
• Access to a treated lawn or unfamiliar plants → prevent that access straight away

Frequently asked questions

Is it dangerous if my dog or cat eats grass?

Not on its own. Do watch out for treated lawns, poisonous ornamental plants, and slug slime, because of the lungworm risk in dogs.

Should I stop my pet from eating grass?

There is no need, unless the behaviour is excessive or compulsive, or takes place on unsafe ground. Occasional grazing is normal behaviour.

Does cat grass actually help?

Yes. A pot of specially grown cat grass gives your cat a safe, controlled way to graze, without the risks of outdoor grass such as pesticides or slug slime.

When should I go to the vet?

If there is persistent vomiting, blood in vomit or stool, lethargy, weight loss, or if your pet also eats non-food items.

Summary

Eating grass is largely normal foraging behaviour in dogs and cats and is usually not a sign of illness: scientific research has largely debunked the popular "they want to be sick" myth. Do watch out for frequent vomiting, diarrhoea, lethargy, or eating non-food items, and bear in mind the risk of lungworm via slug slime as well as treated lawns. With a balanced diet, enough mental stimulation, and an up-to-date worming schedule, you can keep this behaviour within healthy limits.

Still unsure about your pet's behaviour, or want extra support for their digestion? Get in touch with your vet, or take a look at the puzzle toy mentioned above.

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