Most dogs eat grass because it is normal dog behavior — not because they are sick and not because they are trying to make themselves vomit. That is the conclusion of the largest survey ever done on the subject, and of the only controlled feeding experiment on grass eating in dogs. The popular "upset stomach" explanation is the one thing the evidence does not support well.
Here is what researchers have actually measured, which explanations hold up, which don't, and the risks that matter far more than the behavior itself: what's on the grass.
What the research found
In 2008, Karen Sueda, Benjamin Hart and Kelly Cliff of UC Davis published Characterisation of plant eating in dogs in Applied Animal Behaviour Science. They ran three surveys, and they designed them specifically to test the illness-and-vomiting hypothesis.
- 25 veterinary students with dogs: every one reported their dog ate grass. None had ever seen signs of illness beforehand. Only 8% said their dog regularly vomited afterward.
- 47 owners at a veterinary teaching hospital: 79% had seen their dog eat plants, grass most often. Among the 33 who answered the follow-up questions, signs of illness beforehand were reported for four dogs and occasional vomiting afterward for six.
- 1,571 usable web surveys from plant-eating dog owners: 68% ate plants daily or weekly. Only 8% frequently showed signs of illness before eating plants, and 22% regularly vomited afterward.
Hart's own summary of the work is blunt: grass eating is a common behavior in normal dogs, unrelated to illness, and "vomiting seems to be incidental to, rather than caused by, plant eating."
The survey also found no link between grass eating and diet. Dogs fed table scraps or raw food were no more likely to eat grass than dogs on commercial diets, and dogs on lower-fiber diets were no more likely to eat grass than dogs on higher-fiber ones.
The one controlled experiment
Surveys rely on owner memory, so the 2007 University of New England study by Bjone, Brown and Price matters. Twelve healthy, dewormed dogs were offered pots of couch and kikuyu grass three times a day for six days, with every episode filmed and counted.
The result that undercuts the emetic theory: across 709 separate grass-eating events, there were five vomiting events, involving three dogs. If grass were a self-administered emetic, that number should be far higher.
What the experiment did find was a hunger pattern. Dogs spent significantly more time eating grass before their daily kibble meal than after it (2.67 vs 0.53 minutes), and grass eating declined through the day. The longer since the last meal, the more grass. The authors' reading: dogs may simply treat grass as a minor food item.
Myth vs. what the evidence shows
| What you'll read online | What the evidence shows | Source |
|---|---|---|
| "Dogs eat grass to make themselves vomit." | Not supported. 5 vomiting events across 709 grass-eating episodes in the controlled study; only 8% of owners in the large survey saw illness signs beforehand. | Bjone et al. 2007; Sueda et al. 2008 |
| "Grass eating means your dog's stomach is upset." | Weakly supported. It happens in a minority. Dogs that did look ill beforehand were more likely to vomit after — so it isn't zero, just uncommon. | Sueda et al. 2008 |
| "Your dog needs more fiber in its food." | Not supported by the survey data. No association between dietary fiber level and grass eating. | Sueda et al. 2008 |
| "Raw or homemade diets stop grass eating." | No indication of any difference by diet type. | Sueda et al. 2008 |
| "It's an instinct for purging worms." | Interesting hypothesis, not proven in dogs. Based on leaf-swallowing in wild chimpanzees plus the finding that younger dogs eat plants more. Hart presents it explicitly as a hypothesis needing more study. | Hart, dvm360, 2009 |
| "A healthy dog wouldn't do this." | Backwards. All 12 vet-checked healthy dogs in the controlled trial ate grass readily; every one of the 25 vet students' dogs did too. | Bjone et al. 2007; Sueda et al. 2008 |
The explanations, ranked by evidence strength
Best supported: normal inherited foraging behavior
Grass and plant material turns up in a large share of wolf and cougar scat and stomach samples. Combined with how universal the behavior is in healthy pet dogs, the leading interpretation is that it is an innate predisposition carried over from wild ancestors, not a symptom.
Moderately supported: hunger and food-seeking
The pre-meal spike in the controlled trial is a real, measured effect in a small sample of 12 dogs. It suggests appetite plays a role, but it comes from one study and should not be over-read.
Plausible but thinly evidenced: boredom, habit, attention
Veterinary behavior sources commonly list under-stimulation as a contributor, and owners often report the behavior clusters in dull, unsupervised yard time. There is no controlled study quantifying this in dogs. Treat it as a reasonable working theory, not a finding.
Popular but weakly supported: gastrointestinal discomfort
Real in a minority of dogs, wrong as a general explanation. Hart notes that subclinical gastric upset may occasionally trigger it, and dogs showing illness signs beforehand were more likely to vomit after — but that is a small subset, not the norm.
Speculative: parasite purging
The chimpanzee leaf-swallowing parallel is documented in primates. Extending it to dogs is a hypothesis, and the researchers who proposed it say so.
Not supported: dietary deficiency
The survey data found no diet or fiber relationship. No food or supplement has been shown to stop grass eating, and any product claiming to is making a claim the research does not back.
The risks that actually matter
The behavior is usually harmless. What the grass is carrying often isn't.
- Lawn herbicides. A 2013 Purdue study (Knapp et al., Science of the Total Environment) detected herbicides including 2,4-D in the urine of dogs from 19 of 25 households after lawn treatment — and in 14 of 25 before treatment, plus 4 of 8 untreated households, consistent with chemical drift from neighboring lawns. Herbicide residues were also commonly found on grass from untreated lawns. Hart's own client advice is to keep grass-eating pets off chemically treated lawns.
- Slug and snail bait. Metaldehyde is the active ingredient in the majority of slug baits worldwide and causes agitation, tremors, seizures and dangerous hyperthermia. There is no antidote. This is an emergency, not a wait-and-see.
- Foxtails and grass awns. These barbed seed heads travel one way only — deeper. They migrate into ears, noses, between toes and occasionally into body cavities, typically causing abscesses. Peak season is summer, year-round in warm climates.
- Intestinal parasites. Grass fouled by other animals' feces can carry roundworm and hookworm eggs.
- Lungworm. Angiostrongylus vasorum uses slugs and snails as intermediate hosts. Dogs can pick it up by eating a gastropod hidden in grass, or the slime trail left on it. It is regionally distributed but potentially fatal.
A decision framework
Normal — just monitor
Occasional or even daily grazing, no illness before, no vomiting after, normal appetite, normal stools, normal energy. Nothing to fix.
Manage with environment and enrichment
Grazing that looks bored or repetitive, or happens on ground you don't control. Stop lawn treatments or check what your service uses and its re-entry interval, keep dogs off freshly treated grass including neighbors', clear slug bait from the yard, check paws, ears and armpits after walks in dry seedy grass in summer, keep parasite prevention current, and add a sniff walk or food puzzle before the yard shift.
Book a vet appointment
Grass eating that starts suddenly or escalates sharply; repeated vomiting or retching; reduced appetite; weight loss; ongoing diarrhea; lethargy; or eating non-food items such as dirt, rocks, fabric or plastic. These are reasons for a veterinary assessment, not a diagnosis — several unrelated conditions can produce them.
Urgent care now
Suspected slug bait ingestion; tremors, twitching or seizures; blood in vomit or stool; repeated unproductive retching with a bloated or tight abdomen; sudden collapse; violent sneezing, head shaking or squinting after being in tall dry grass, which can signal a lodged awn.
Frequently asked questions
Should I stop my dog from eating grass?
Not for its own sake. Healthy dogs eating clean, untreated grass are doing something normal. Redirect based on where the grass is, not the behavior itself.
Does grass eating mean my dog has an upset stomach?
Usually not. Only 8% of owners of 1,571 plant-eating dogs frequently saw illness signs beforehand. When a dog does look unwell first, that is worth a vet's attention.
Why does my dog eat grass and then throw up?
About 22% of dogs in the large survey regularly vomited afterward, but the controlled trial recorded only five vomiting events across 709 grass-eating episodes. Frequent vomiting is a reason to see a vet regardless of the grass.
Will more fiber or a diet change stop it?
The survey found no link between fiber intake or diet type and grass eating. No diet or supplement has been shown to stop it.
Is eating grass every day a problem?
68% of dogs in the survey ate plants daily or weekly. Frequency alone is not the warning sign; a sudden change in frequency, or grass eating alongside vomiting, appetite loss or lethargy, is.
UK guidance
For UK-specific advice, read PDSA's guidance on PDSA Pet Health Hub, written for UK pets. If your dog's signs are new, persistent or worsening, speak with a vet; this article is general information, not a diagnosis.
Sources
- Sueda KLC, Hart BL, Cliff KD. Characterisation of plant eating in dogs. Applied Animal Behaviour Science 2008;111:120–132 — ScienceDirect
- Hart BL. Why do dogs and cats eat grass? dvm360, Dec 2009 — dvm360.com
- Bjone SJ, Brown WY, Price IR. Grass eating patterns in the domestic dog, Canis familiaris. Recent Advances in Animal Nutrition in the UK 16 (2007) — une.edu.au
- Knapp DW et al. Detection of herbicides in the urine of pet dogs following home lawn chemical application. Science of the Total Environment 2013;456–457:34–41 — PubMed
- Merck Veterinary Manual — Metaldehyde Poisoning in Animals — merckvetmanual.com
- Merck Veterinary Manual — Lungworm Infection in Dogs — merckvetmanual.com
- Companion Animal Parasite Council — Angiostrongylus vasorum — capcvet.org
- Pet Poison Helpline — Foxtail Grass Foreign Body — petpoisonhelpline.com
This article is for general information and does not replace veterinary care. If your dog is unwell, or if the behavior has changed suddenly, contact your veterinarian. In a suspected poisoning, contact your vet or an animal poison control service immediately.