The short answer: change your dog's food gradually over seven to ten days, mixing the new food in with the old and increasing it in steps. Most dogs do fine on a 25 per cent, 50 per cent, 75 per cent, 100 per cent progression across that window. Sensitive dogs need two to three weeks and smaller steps. Loose stools for a day or two in the middle are common. Blood, vomiting, refusal to eat, or a dog who seems genuinely unwell is not part of the process and needs a vet.
Almost every food problem we get asked about starts the same way. Somebody buys a better food, pours a bowl of it that evening, and by Thursday the dog has had three bad mornings in the garden and the owner has concluded the food does not suit. Nine times out of ten the food was fine. The handover was the problem.
Why a sudden change upsets a dog's stomach
A dog's gut runs on a settled population of bacteria that has spent months adapting to one recipe. Change the protein, the fat level, the fibre and the carbohydrate source overnight and that population has to rearrange itself around a new job. While it does, digestion gets sloppy. That is the loose stool.
The fix is not a gentler food. The fix is giving the gut a fortnight's notice instead of none.
Two other things change at the same time and are worth knowing about. Kibble density varies a lot between brands, so the same scoop can be a different number of calories, and a dog who suddenly eats more is a dog with a looser stool for reasons that have nothing to do with tolerance. And fat content varies more than most owners realise. Going from a fifteen per cent fat food to a twenty per cent fat food is a real jump, even if both bags look similar on the shelf.
The ten-day switch, in actual spoonfuls
Work in portions of the daily ration rather than in bowls, because most dogs eat twice a day and the ratio needs to hold at both meals.
| Days | Old food | New food | What to watch |
|---|---|---|---|
| 1 to 3 | 75% | 25% | Appetite. Most dogs eat it without comment. |
| 4 to 6 | 50% | 50% | This is where loose stools usually show up if they are going to. |
| 7 to 9 | 25% | 75% | Stools should be firming up again by day eight or nine. |
| 10 onwards | 0% | 100% | Give it another two weeks before you judge coat, energy or itch. |
Two rules make this work better than the table alone. Do not advance a stage while the stool is still loose. Hold at the current ratio for two or three extra days until it firms, then move on. And do not change anything else during the switch. No new treats, no new chew, no table scraps, no supplement starting on day four. If something goes wrong you want one variable, not four.
If your dog has a sensitive stomach, slow down
For a dog with a history of digestive trouble, stretch the same progression over two to three weeks and use smaller increments: ten per cent, then twenty five, then forty, then sixty, then eighty, then all of it. It feels excessive. It is also the difference between a quiet fortnight and a fortnight of carpet cleaner.
Our usual recommendation for a genuinely sensitive dog is to switch onto a single-protein, grain-free recipe so that if something does flare later, there are few enough ingredients to reason about. Happy Dog Africa is ostrich and potato, at £8.69 for 1kg, £33.79 for 4kg and £67.49 for 12.5kg. Happy Dog Montana is horse and potato, £8.69 for 1kg and £61.79 for 10kg. Both use a protein the average British dog has never eaten, which matters if you suspect the old food was the problem.
There is more on choosing between them in our guide to the best dog food for sensitive stomachs in the UK.
Switching from dry to wet, or wet to dry
The same ten-day ratio applies, with one extra thing to get right: you are also changing the water content of the meal, from roughly ten per cent in kibble to around eighty per cent in a tin. Stools get softer simply because there is more moisture going in. That is normal and it settles.
The easiest version of this switch keeps the protein the same on both sides. Our range is built to allow that. Pure Ostrich at £4.79 for a 400g tin is the tinned partner to Africa. Pure Sensitive Horse at £3.85 for 400g pairs with Montana. Pure Sea Fish at £3.85 pairs with Karibik, the deep-sea fish kibble at £8.69 for 1kg and £63.69 for 11kg. Changing texture while holding the protein steady is a much smaller ask of a gut than changing both at once.
If you have not decided which way to go, our comparison of wet versus dry dog food covers the cost and the practicalities side by side.
Within one brand is easier than across two
Moving between recipes in the same range is usually a four or five day job rather than a ten day one, because the fat level, the kibble size and the carbohydrate base are often shared across the line. Going from Happy Dog France at £63.69 for 11kg to Happy Dog Canada at the same price is a smaller change than either of them to a completely different brand.
Across brands, assume the full ten days and read both labels before you start. Our guide to reading a dog food label explains which numbers on the back actually tell you something.
Buy the small bag first. It is the whole trick.
Here is the part that saves real money. A 1kg bag of any Happy Dog Sensible recipe is £8.69. A 12.5kg sack of Africa is £67.49. If you buy the sack and your dog turns out not to like ostrich, you own sixty seven pounds of a food that will sit in the utility room until you give it away.
Buy the 1kg bag, run the first six days of the switch on it, and you will know two things for the price of a coffee round: whether your dog will eat it, and whether it agrees with them. Then buy the sack, where the price per kilo drops from £8.69 to £5.40.
The same logic applies to tins. A single 400g tin is £3.85 to £4.79 depending on the protein. A twelve pack is £35.69 to £45.29. Find out with one tin.
For the arithmetic on how long a bag actually lasts, see how much dry food to feed your dog.
What normal looks like, and what does not
Normal during a switch: a softer stool for a day or two around the halfway mark. Slightly more wind. A dog who picks at the new food on day one and then gets on with it. One day of eating less than usual.
Not normal, and worth a call to your vet: blood or mucus in the stool, repeated vomiting, diarrhoea that lasts more than forty eight hours, a dog who will not eat at all for a day, lethargy, a painful or bloated abdomen, or any of this in a puppy, a senior dog or a dog with an existing health condition, where the margin for dehydration is much smaller.
Trust that instinct. A food switch is not a reason to wait and see with a dog who looks unwell.
When a slow switch is the wrong answer
There are two exceptions. If your vet has put your dog onto a prescription or therapeutic diet, follow their timetable, which is sometimes immediate and for good reason. And if you are starting a proper exclusion trial for a suspected food reaction, the point is to remove everything else, so a long overlap with the old food defeats the exercise. Our step-by-step elimination diet guide sets that out properly, because it is a different job with different rules.
Puppies and older dogs
Puppies have less reserve than adults and dehydrate faster, so go slower, not faster, and ring your vet sooner. The only puppy-formulated food we stock is AATU Salmon Dry Food for Puppies at £22.79 for 1.5kg and £55.99 for 5kg. If you are moving a puppy off what the breeder was feeding, get the name of that food and overlap properly rather than guessing.
Older dogs are often on medication, and a change in diet can change how a drug is absorbed or how enthusiastically a tablet hidden in food gets eaten. Mention the switch at the next appointment.
The four mistakes we see most
- Switching on a Friday. If something goes wrong you want your vet open. Start on a Monday.
- Mixing two foods in one bowl and letting the dog pick. Dogs are good at eating round the new bits. Mix it properly, and dampen the kibble slightly if you have to.
- Judging the food at day three. Day three is the worst day. Coat, energy and skin take four to eight weeks to show a difference, not four days.
- Starting a supplement, a new treat and a new food in the same week. One change at a time, or you learn nothing from the result.
If the problem is that they will not eat it at all
That is a different problem from a stomach problem, and a very common one. Our guide on what to do when a dog will not eat covers the practical ways round it, including using a quarter of a matching tin as a topper rather than dragging a new ingredient into the diet mid-switch.
Browse the full dry dog food and wet dog food ranges, or the whole dog food collection. Free UK delivery on orders over £30, which a 4kg bag clears on its own.
Frequently asked questions
How long should it take to switch a dog's food?
Seven to ten days for most dogs, moving from a quarter new food to half, to three quarters, to all of it in roughly three-day steps. Two to three weeks for a dog with a sensitive stomach or a history of digestive trouble. Do not advance a step while the stool is still loose.
Can I switch my dog's food straight away?
You can, and some dogs are completely untroubled by it, but a sudden change is the usual cause of the loose stools that get blamed on the new food. The only times to switch immediately are when a vet has told you to.
Is it normal for a dog to have diarrhoea when changing food?
A softer stool for a day or two around the halfway point is common. Watery diarrhoea lasting more than forty eight hours, blood or mucus in the stool, repeated vomiting or a dog who seems unwell is not part of a normal switch and needs a vet.
Do I need to switch gradually between two flavours of the same brand?
Usually a shorter overlap of four or five days is enough, because recipes in one range tend to share a fat level and a carbohydrate base. Check both labels anyway, since single-protein ranges can still differ in fat.
How do I switch from dry food to wet food?
Use the same ten-day ratio, and keep the protein the same on both sides if you can. Expect softer stools from the extra moisture alone. Pure Ostrich pairs with Happy Dog Africa, Pure Sensitive Horse with Montana, and Pure Sea Fish with Karibik.
How long before I can tell whether a new food suits my dog?
Digestion settles within about two weeks. Coat condition, itchiness and energy take four to eight weeks, and an exclusion trial for a suspected food reaction needs eight to twelve weeks on one protein.
What size bag should I buy to try a new food?
The smallest one. Every Happy Dog Sensible recipe comes as a 1kg bag at £8.69, and every Pure recipe comes as a single 400g tin from £3.85, which is enough to run the first half of a switch before you commit to a sack or a case.
Featured in this guide: Happy Dog Africa (from £8.69) · Happy Dog Montana (from £8.69) · Happy Dog Karibik (from £8.69) · Pure Ostrich (from £4.79) · Pure Sensitive Horse (from £3.85) · Pure Sea Fish (from £3.85) · AATU Salmon for Puppies (from £22.79). Free UK delivery over £30.
General guidance, not veterinary advice. If your dog is unwell, or is on medication or a prescription diet, speak to your vet before changing their food.