Green tea extract is a concentrate of catechins, the polyphenols in the leaf of Camellia sinensis. The most abundant one is EGCG (epigallocatechin gallate). A handful of small studies in dogs point to changes in blood fats, insulin sensitivity and bad breath, but none is enough to claim a benefit. What has been studied best in dogs is its safety. Given with food and at moderate doses it was well tolerated, and at high doses given on an empty stomach it damaged the gut and liver of laboratory dogs.
Catechins, caffeine and how extracts differ
Green tea is made from leaves that are not fermented, which stops their polyphenols from oxidising. Most of those polyphenols are catechins, according to the 2018 scientific opinion on their safety by the European Food Safety Authority (EFSA). The same review shows how much leaves vary: EGCG in the dried leaf ranges from 1,600 to 20,320 mg per 100 g, a thirteen-fold difference. The leaf also contains between 2.9 and 4.2% caffeine.
An extract concentrates these substances using water, alcohol or other solvents, and what ends up in it depends on the leaf and the method. Decaffeinated extracts exist. The United States Pharmacopeia (USP) requires its powdered decaffeinated green tea extract to contain at least 40% EGCG and no more than 0.1% caffeine. The EU has no harmonised legal limit. EFSA mentions a maximum of 1.2% caffeine in the dry matter of decaffeinated extracts, while national limits still apply. If you are comparing products, look for two things on the label: how much EGCG it provides and whether the extract is decaffeinated.
Caffeine matters because dogs are sensitive to it. The MSD Veterinary Manual puts mild signs (vomiting, diarrhoea, marked thirst) at around 20 mg/kg of methylxanthines, the group that includes caffeine and the theobromine in chocolate. Effects on the heart appear from 40 to 50 mg/kg, and seizures from 60 mg/kg. With a decaffeinated extract at 1.2%, a 10 kg dog would have to eat more than 16 g of extract to reach 200 mg of caffeine. Tea leaves and tea bags keep all of theirs.
What it has been tested for in dogs
The studies in dogs are few, small and short, most of them in laboratory dogs, and several test green tea mixed with other ingredients.
Insulin and blood fats
Six obese dogs in a 2008 study received 80 mg/kg of green tea extract a day, just before their single meal, for 12 weeks, and four others served as controls. At the end, the treated dogs had 60% higher insulin sensitivity than at the start and 50% lower triglycerides. That is ten dogs, compared mainly with their own starting point, and the results are blood and gene tests, not weight loss.
Body weight
A 2020 Chinese study split 16 young beagles into four groups of four for 12 weeks: a normal diet, a very high-fat diet, and the high-fat diet with tea polyphenols mixed into the food at 250 or 500 mg per kg of body weight. The dogs on the high-fat diet alone went from 12.2 to 16.6 kg. Those on the higher dose finished at 11.6 kg on average. This is obesity brought on by a very high-fat diet in groups of four dogs, a long way from an overweight pet dog on a weight-loss plan.
Mouth and bad breath
On culture plates, catechins hold back bacteria from the dog's mouth. A 2025 paper found that catechin from green tea killed Porphyromonas gulae and P. gingivalis, two bacteria involved in periodontal disease, and cut the sulphur compounds that cause the smell. Dogs and cats with periodontal disease then ate wet food containing 0.01892% catechin for a month. Their breath improved and P. gulae activity fell, but plaque and gingivitis did not change.
Joint formulas
Green tea turns up in some joint formulas. A randomised, double-blind 2017 trial with 42 osteoarthritic dogs added the extract to the diet in a trio with curcuminoids and hydrolysed collagen, a mix whose result the trial could not split. Over three months the diet reduced pain on manipulation and improved the dogs' ability to get up. It did not change the weight-bearing force measured on a force plate, though, or the blood markers of osteoarthritis.
The antioxidant effect
We have not found any study in dogs that measures a clinical benefit from the antioxidant effect of green tea on its own, even though that effect is the argument made for it most often. The two antioxidants with the longest track record in canine nutrition are vitamin E, which is an essential nutrient, and vitamin C, which the dog's own liver makes.
Doses used in studies
These are the doses used in studies with dogs. They are not a recommendation.
| Study | Dogs | Dose and form | Result |
|---|---|---|---|
| Obesity, 2008 | 10 obese dogs (6 treated) | 80 mg/kg/day of extract, before the meal, 12 weeks | Better insulin sensitivity and lower triglycerides than at the start |
| High-fat diet, 2020 | 16 beagles (8 given tea) | 250 or 500 mg/kg body weight of tea polyphenols mixed into the food, 12 weeks | Less weight gain |
| Mouth, 2025 | Dogs and cats with periodontal disease | Wet food with 0.01892% catechin, one month | Less bad breath; no change in plaque or gingivitis |
| Toxicity, 2006, with food | 6 beagles per sex per dose | 50, 300 or 500 mg/kg/day of a preparation with 91.8% EGCG, in two doses one hour after eating, 13 weeks | No adverse effects |
| Toxicity, 2006, fasted | 4 beagles per sex per dose | 50, 150 or 500 mg/kg/day (80% EGCG) in capsules after at least 15 hours without food, 13 weeks | Deaths and euthanasia for severe illness at 150 and 500 mg/kg; no adverse effects at 50 mg/kg |
| Toxicity, 2009, fasted | 24 treated beagles | 200, 500 or 1,000 mg/kg/day of extract (about 128, 320 and 640 mg/kg of EGCG) in capsules, fasted | 16 of 24 died before the planned end; study stopped at 6.5 months |
Safety: why it matters whether the dog has eaten
In the 2006 safety studies, 500 mg/kg a day split into two doses after a meal caused no adverse effects, while the same dose given in one go on an empty stomach made the dogs ill. The fasted dogs in the 2009 study had vomiting, diarrhoea, excessive drooling and liver lesions. In the follow-up phase, at 200 mg/kg for 13 weeks, vomiting, mild diarrhoea or red material in the faeces, anaemia and mild liver changes appeared again among the nine fasted dogs. The three that got the dose with food had no digestive or liver problems.
According to EFSA, EGCG in the blood can be up to ten times higher when the dog is fasted. That does not explain everything, however. Wu and colleagues pointed out that at the no-effect dose of each study, the fasted dogs actually had less EGCG in their blood than the fed ones. Their hypothesis is that fasting leaves the gut and liver more vulnerable. EFSA added that a fasted liver holds less glycogen.
In people, EFSA concluded that taking 800 mg or more of EGCG a day as a supplement raises transaminases, a sign of liver damage. For a 70 kg person that is about 11 mg/kg. The USP's 2020 review added a warning to its monograph: do not take it on an empty stomach, take it with food, and do not use it if you have a liver problem.
- Give it with food. The serious problems in dogs came from capsules given on an empty stomach, so do not give it on a day your dog is not eating, or during the fast before an anaesthetic.
- Use it in a dog with liver disease only if the vet approves. The gut and liver are among the organs damaged in the toxicity studies, and we have not found studies in dogs with liver disease or in pregnant or lactating bitches.
- Do not give tea, tea leaves or tea bags. If your dog has epilepsy or heart disease, use decaffeinated extracts only and mention it to the vet.
- Ask your vet first if your dog takes medication every day or already gets another product containing green tea, and let them set the dose. No interactions have been studied in dogs.
Stop the product and call the vet if you see vomiting, diarrhoea, blood in the faeces, excessive drooling, lethargy, loss of appetite or a yellowish tinge to the gums or eyes. These are the signs seen in the dogs in the toxicity studies, or the usual signs of a liver problem.
Sources
- Scientific opinion on the safety of green tea catechins. EFSA Journal, 2018.
- Exposure and toxicity of green tea polyphenols in fasted and non-fasted dogs. Toxicology, 2009.
- Safety studies on epigallocatechin gallate (EGCG) preparations. Part 2: dermal, acute and short-term toxicity studies. Food and Chemical Toxicology, 2006.
- Green tea extract-induced lethal toxicity in fasted but not in nonfasted dogs. International Journal of Toxicology, 2011.
- United States Pharmacopeia (USP) comprehensive review of the hepatotoxicity of green tea extracts. Toxicology Reports, 2020.
- Effects of green tea on insulin sensitivity, lipid profile and expression of PPARalpha and PPARgamma and their target genes in obese dogs. British Journal of Nutrition, 2008.
- Tea polyphenols attenuate liver inflammation by modulating obesity-related genes and down-regulating COX-2 and iNOS expression in high fat-fed dogs. BMC Veterinary Research, 2020.
- Oral treatment with catechin isolated from Japanese green tea significantly inhibits the growth of periodontal pathogen Porphyromonas gulae and ameliorates the gingivitis and halitosis caused by periodontal disease in cats and dogs. International Immunopharmacology, 2025.
- A randomized, double-blind, prospective, placebo-controlled study of the efficacy of a diet supplemented with curcuminoids extract, hydrolyzed collagen and green tea extract in owner's dogs with osteoarthritis. BMC Veterinary Research, 2017.
- Chocolate Toxicosis in Animals. MSD Veterinary Manual.