Why Does My Dog Eat Grass and Vomit?
Key takeaways
- A UC Davis study of 1,571 dog owners found grass eating is common and normal. Only about 10% of dogs were ill before eating grass.
- Fewer than one in four dogs vomit afterward. The researchers concluded vomiting is incidental, not caused by the grass.
- Likely reasons include instinct, fiber seeking, boredom, and learned behavior. See a vet if the grazing is frantic or constant.
Image Credits: Wikimedia Commons
Your dog eats grass and sometimes vomits. The usual story says the dog is treating itself: eat grass, feel sick, throw up, feel better.
The research says otherwise. The largest study on this question surveyed 1,571 owners of plant-eating dogs at the University of California, Davis. It found grass eating is common in normal dogs and largely unrelated to illness. The vomiting, when it happens, looks incidental.
That does not mean the behavior is meaningless. It means the meaning differs from the folk tale. Several real explanations fit the data better.
This article covers why your dog eats grass and vomits: the myth versus the data, the UC Davis study's findings, the likeliest real reasons, when grass eating signals a problem, and what to do about it.
The myth versus the data
The folk belief is simple: dogs eat grass to make themselves vomit when they feel sick. It feels true because some dogs do vomit right after grazing.
The numbers break it. In the UC Davis study by Sueda, Hart, and Cliff, only about 10% of dogs showed illness before eating grass. About 90% were apparently fine.
After eating grass, fewer than one in four dogs vomited. VCA Animal Hospitals reports similar figures: fewer than 25% of dogs vomit after eating grass, and only about 10% show illness beforehand. VCA's conclusion is blunt: dogs are unlikely to use grass as self-medication.
A separate experiment tested the self-medication idea directly. In a 2010 study in Applied Animal Behaviour Science, Bjone and colleagues fed dogs a diet with fructo-oligosaccharides to cause mild diarrhea. Then they measured grass eating.
The result ran against the theory. Dogs ate much less grass with loose stools than with normal stools. If grass were medicine for gut trouble, sick dogs should have eaten more, not less.
The UC Davis study: 1,571 owners
The UC Davis work is still the biggest dataset on this question. Its six findings are worth stating plainly.
First, grass eating is common in normal dogs. A USA Today report puts the figure at 68% of dogs eating grass or plants daily to weekly.
Second, it is often unrelated to illness. The vast majority of dogs showed no sickness before grazing.
Third, dogs do not regularly vomit after eating plants. The vomiting rate was below one in four.
Fourth, younger dogs ate plants more often than older dogs. The young ones were also less likely to look ill beforehand or vomit afterward. That age pattern matters. It points away from sickness and toward instinct.
Fifth, diet type had no link to grass eating. Dogs on commercial food, raw diets, and table scraps grazed at similar rates. This weakens the simple nutritional-deficiency story.
Sixth, the researchers concluded that plant eating is common in normal dogs and unrelated to illness. Vomiting seems incidental, not caused by plant eating.
One honest caveat: dogs that showed illness before eating grass were more likely to vomit afterward. So a small group of dogs may graze urgently when unwell and then throw up. That covers a minority of cases, not the behavior as a whole.
Why they probably do it
With self-medication mostly ruled out, several theories fit the evidence better.
Instinct from wild ancestors. Wolves eat grass. Dogs may keep the habit as an evolutionary holdover. One version of the theory, linked to researcher Benjamin Hart's 2008 work, says grass eating helped purge gut parasites. This is like wild chimpanzees swallowing rough leaves. The finding that younger dogs graze more supports this. Young animals with developing immune systems are most at risk from parasites.
Fiber seeking. Grass is roughage, and at least one case supports the fiber theory. A miniature poodle ate grass and vomited daily for seven years. It stopped completely after switching to a high-fiber diet. This case appeared in the Journal of Veterinary Medical Science. But the UC Davis finding of no diet link tempers this. It explains some dogs, not all.
Learned behavior. Dr. Katherine Houpt, professor emeritus at the Cornell College of Veterinary Medicine, told USA Today that grazing can be learned. Puppies of mothers who ate grass are more likely to copy it.
Curiosity and scent. Dogs have a vomeronasal organ between the nose and mouth. It is a secondary scent detector. Houpt notes that holding grass in the mouth helps a dog analyze who passed by or urinated there. Some grazing is investigation, not eating.
Boredom. Dogs left alone in yards with little to do graze the way bored people snack. More play and enrichment often reduces the habit. That points to behavior, not biology.
They like it. The simplest theory deserves its slot. For some dogs, grass tastes or feels satisfying. Not everything dogs do has a deep reason.
Image Credits: Wikimedia Commons
When grass eating is a problem
Normal grazing is casual: a few blades here and there on a walk or in the yard, with the dog otherwise happy and healthy.
Call your vet if the pattern changes. Frantic, urgent grass eating is a warning sign. This is especially true with lip licking, drooling, or repeated swallowing. These can signal nausea or acid reflux. Constant new grazing, or grazing with appetite loss, tiredness, or diarrhea, deserves a checkup.
Also consider what is on the grass. Lawns treated with herbicides, pesticides, or fertilizers turn innocent grazing into a poisoning risk. If your dog grazes, know what your lawn service uses. Keep the dog off recently treated grass.
Pica is a separate concern. If your dog compulsively eats non-food items beyond grass, like rocks, plastic, fabric, or soil, that is a different condition. It can indicate nutritional deficiency, anemia, or a behavioral disorder. It needs vet diagnosis.
What to do about it
For the ordinary grazer, the honest answer is that you mostly need do nothing. It is normal behavior in a normal dog.
If it bothers you, a few steps help. Make sure the diet has enough fiber. The poodle case shows diet can matter for individual dogs. Increase exercise and mental enrichment to rule out boredom grazing.
Train a reliable "leave it" for walks near treated lawns or unknown plants. Some ornamental plants are toxic. The danger is not the grass but what grows beside it.
Do not punish the grazing. It is not misbehavior. Scolding a dog for following an instinct creates anxiety without changing the instinct.
Frequently asked questions
Do dogs eat grass to make themselves vomit?
Mostly no. Only about 10% of grass-eating dogs show illness beforehand. Fewer than 25% vomit afterward, per the UC Davis study of 1,571 owners and VCA Animal Hospitals. A small minority do seem to graze urgently when unwell. That is the exception.
Is it safe to let my dog eat grass?
Usually yes, with one big condition: the grass must be free of herbicides, pesticides, and fertilizers. Casual grazing on clean grass is normal. Keep your dog off recently treated lawns and away from toxic ornamental plants.
Why does my dog eat grass and throw up yellow foam?
Yellow foam is usually bile. Bile collects in an empty stomach. Dogs that graze and vomit bile before the morning meal may react to an empty, irritated stomach. This is about the empty stomach, not the grass. Try a small late-evening snack. Mention it to your vet if it repeats.
Should I change my dog's food if it eats grass?
Not necessarily. The UC Davis study found no link between diet type and grass eating. But if the grazing is heavy and new, a diet review with your vet is reasonable. In at least one published case, switching to high-fiber food stopped a seven-year grass-eating habit.
The bottom line
Your dog eats grass because dogs eat grass. The biggest study on the question found it common, normal, and mostly unrelated to illness. Vomiting is an incidental side effect, not the goal.
The likeliest reasons are instinct, fiber, curiosity, boredom, and simple enjoyment. Let the casual grazer graze on clean grass. Save the vet visit for the frantic, sudden, or constant grazer, and for any lawn that has seen chemicals.
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About the author: Rive News is the research desk behind these explainers, covering practical questions with verified facts and named sources.
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