How to switch dog food safely — without an upset stomach. Changing a dog's food is one of the most common causes of diarrhoea, vomiting, and a dog that will not eat, and it is also one of the easiest to prevent: switch gradually, over a week, mixing the old and new food in increasing proportions, so the dog's digestion can adjust. This guide is the switching plan in words: the 7-day plan, why gradual switching matters, what to do when the switch goes wrong, and when a food change is a reason to call the vet. It applies to any food change — a new brand, a new life stage, a new form, or a therapeutic diet.
The short version is in the table below: 75% old and 25% new for two days, 50% and 50% for two days, 25% and 75% for two days, then 100% new on day 7. That is all there is to it — and it is worth doing, because a sudden switch upsets digestion, and an upset stomach in a dog that has just changed food is usually blamed on the new food, when the real cause is the speed of the change.
The 7-day switching plan
| Day | What to feed |
|---|---|
| Day 1–2 | 75% old food, 25% new food |
| Day 3–4 | 50% old food, 50% new food |
| Day 5–6 | 25% old food, 75% new food |
| Day 7 | 100% new food |
Mix the old and new food in the same bowl, in the proportions above, and feed the mixture at the usual times, in the usual amount. Do not change the amount at the same time as the food: keep the daily amount the same, so the only thing changing is the food itself, and you can tell whether the dog is reacting to the food or to the amount. Measure both foods with scales, so the proportions are accurate — a guessed 25% is often 50%, and a guessed 50% is often 75%. Fresh water should always be available, and the dog's stools should be watched: a small change in firmness is normal during a switch, but diarrhoea, vomiting, or a dog that will not eat are reasons to slow down or to call your vet.
If the dog picks out the new food and leaves the old, or picks out the old and leaves the new, mix the foods more thoroughly, or moisten the mixture with a little warm water so the two foods stick together. If the dog will not eat the mixture at all, go back a step — more old food, less new — and move more slowly. A dog that has never eaten the new food before may need longer than a week: some dogs need two weeks, and that is fine, as long as the proportions move in the right direction. The feeding schedule generator builds a timetable from your dog's age and size, and the dog food calculator works out the daily amount for any food.
Why gradual switching matters
A dog's gut contains bacteria that digest its food, and those bacteria are adapted to the food the dog is eating: they digest that food well, and they digest a different food less well. When the food changes suddenly, the bacteria have not had time to adapt, and the result is an upset stomach — loose stools, gas, vomiting, or a dog that will not eat. Switching gradually gives the gut bacteria time to adapt, so the new food is digested well from the start. That is why the 7-day plan works, and why a sudden switch so often fails.
Gradual switching also lets you tell whether the new food suits the dog: if the dog develops loose stools on day 3, when the mixture is half and half, you can go back to 75% old and 25% new and move more slowly, and you will know that the dog needs more time. If you switch suddenly and the dog develops loose stools, you cannot tell whether the food does not suit it, or whether the switch was too fast — and the usual response is to blame the food and switch again, suddenly, to another food, which upsets the stomach again. Slow switching breaks that cycle: it gives the dog time, and it gives you information.
When a switch goes wrong
A switch goes wrong when the dog develops diarrhoea, vomiting, or a loss of appetite that lasts more than a day, or when it will not eat the new food at all. The first step is to slow down: go back to the last proportion the dog tolerated, stay there for two or three days, and then move forward more slowly — a 10% step every two days, rather than a 25% step. If the dog still will not tolerate the new food, stop the switch and go back to the old food, and ask your vet whether the new food is right for the dog — it may be that the food does not suit it, or that the dog has a health problem that needs investigating.
A dog that develops diarrhoea or vomiting during a switch needs fresh water always available, and it should be watched closely: a young animal, a senior, or a dog with a health condition can become dehydrated fast, and it needs a vet check sooner than a healthy adult. Do not give medicine for diarrhoea or vomiting without veterinary advice, and do not starve the dog to settle its stomach — a dog that will not eat for more than a day needs a vet check, whatever the cause. The how much food should a dog eat guide gives the amounts for senior dogs, and the when is a dog considered a senior guide explains the senior boundary.
Switching for a reason: life stage, health, and preference
Most food changes have a reason: a puppy moving to adult food, an adult moving to senior food, a dog moving to a therapeutic diet for a health condition, or a dog that will not eat its current food. Each reason has its own timing: a life-stage change should be made when the vet says the dog is ready, not at a birthday; a therapeutic diet should be started on veterinary advice, and it should not be mixed with other foods, because that defeats the treatment; and a change for preference should be made slowly, because a dog that is offered a new food every time it refuses one learns to refuse food.
A therapeutic diet is a special case: it is a medicine in food form, and it should be fed exactly as the vet prescribes, without mixing in other foods or treats that could interfere with it. If the dog will not eat a therapeutic diet, ask the vet for another form of the same diet — a wet form instead of dry, or a different flavour — rather than going back to the old food. And if the dog has a health condition, ask the vet before changing its food at all: some conditions need a specific diet, and a change made by guesswork can make the condition worse. The grain-free dog food pros and cons guide explains one label that is often misunderstood when choosing a new food.
Common mistakes when switching food
The most common mistake is switching suddenly: a sudden switch upsets digestion, and the upset stomach is then blamed on the new food, when the real cause is the speed of the change. The second is changing the amount at the same time as the food, so the owner cannot tell whether the dog is reacting to the food or to the amount. The third is switching foods often: a dog that changes food every few weeks never settles, and its digestion never settles either — choose a food, feed it for at least a month, and judge it by the dog. The fourth is giving up too soon: a dog that is offered a new food and refuses it once is not a dog that will never eat it — go back a step, move more slowly, and give it time. And the fifth is ignoring the stools: watch them during a switch, because they are the earliest sign of whether the new food suits the dog.
For related reading, see the dog food calculator, the feeding schedule generator, the grain-free dog food pros and cons guide, the how many calories should a dog eat per day guide, the dog feeding chart by weight guide, the how to switch cat food safely guide.
Frequently asked questions
How long does it take to switch dog food?
About a week for most dogs: 75% old and 25% new for two days, 50% and 50% for two days, 25% and 75% for two days, then 100% new on day 7. Some dogs need two weeks, and that is fine, as long as the proportions move in the right direction. Do not switch suddenly — a sudden switch upsets digestion.
Can I switch dog food cold turkey?
It is not a good idea: a sudden switch upsets digestion, because the gut bacteria have not had time to adapt to the new food, and the result is often diarrhoea, vomiting, or a dog that will not eat. Switch gradually over a week, mixing the old and new food in increasing proportions.
What if my dog will not eat the new food?
Go back a step — more old food, less new — and move more slowly, a 10% step every two days rather than a 25% step. Mix the foods thoroughly, or moisten the mixture with a little warm water. If the dog still will not eat the new food, ask your vet whether it is right for the dog.
What if my dog gets diarrhoea during the switch?
Slow down: go back to the last proportion the dog tolerated, stay there for two or three days, and then move forward more slowly. Keep fresh water always available. If the diarrhoea lasts more than a day, or the dog is young, old, or has a health condition, call your vet.
Should I change the amount when I change the food?
No — keep the daily amount the same, so the only thing changing is the food itself, and you can tell whether the dog is reacting to the food or to the amount. Work out the daily amount from the RER formula for the dog's weight and life stage, or use the dog food calculator.
How often should I change my dog's food?
Rarely. Choose a food, feed it for at least a month, and judge it by the dog — steady weight, a good coat, firm stools, and a vet who is happy. A dog that changes food every few weeks never settles, and its digestion never settles either. Change food for a reason — a life stage, a health condition, or a food the dog will not eat — not out of habit.
Results on this page are estimates for general information only and are not veterinary advice. If your pet is ill, underweight, overweight, pregnant, or has a health condition, consult your veterinarian before changing diet or care.