Posted by Stephen Crowther
on May 07, 2026
If you've ever changed your dog's food overnight and spent the next morning cleaning the kitchen floor, you already know why this guide exists. Switching too fast is one of the most common avoidable causes of dog diarrhoea in the UK — and one of the easiest things to get right once you know the pattern.
This guide is for UK owners who have decided (or been told by a vet) to move their dog onto a new food. Your puppy has hit their first birthday, your senior dog needs a softer kibble, you're trying to fix loose poos, you're moving to fresh from dry, or your usual brand has changed its recipe. Whatever the reason, the answer is the same: take it slowly, watch the poos, and adjust if things go wrong.
The 30-second answer
Mix a small amount of the new food in with the old, and increase the new food's share by roughly 25% every two days over a week. So days 1–2 are 25% new / 75% old, days 3–4 are 50/50, days 5–6 are 75% new / 25% old, and from day 7 you're on 100% new food. UK charities including PDSA, Blue Cross and Dogs Trust all give a version of this same gradual swap.
If your dog has a sensitive stomach, is recovering from a tummy upset, has food allergies, or is being moved onto a prescription diet, double the timeline to 14 days and adjust in 10–15% steps. If at any point your dog gets loose poos, vomits, or stops eating, pause the transition — don't push through.
Why a slow swap matters
A dog's gut isn't just a tube — it's a living ecosystem. The bacterial population that breaks down their food (the gut microbiome) is calibrated to whatever they've been eating, and a sudden swap to a new protein, fat profile, or fibre source throws that balance out. The result is the predictable list of "I just changed his food" symptoms: loose stools, extra wind, occasional vomiting, gurgly tummies, and a temporarily fussier eater.
A slow transition gives the microbiome time to adapt. PDSA puts it simply: changes should happen "over a couple of weeks" to avoid tummy upsets, and an abrupt switch is only appropriate when a vet has specifically asked for one (for example, on a prescription diet where waiting is riskier than swapping).
The standard 7-day transition (most healthy adult dogs)
This is the plan that fits the vast majority of UK dogs in normal health. Aim for two meals a day at the same times you usually feed, with the proportions below mixed in the same bowl.
| Days | Old food | New food | What to watch |
|---|---|---|---|
| 1–2 | 75% | 25% | Stool firmness, appetite, energy |
| 3–4 | 50% | 50% | Any loose poos? Pause if yes |
| 5–6 | 25% | 75% | Settled poos, eating normally |
| 7 onward | 0% | 100% | Switch complete — keep watching for a week |
A few practical notes the manufacturer guides usually leave out:
- Weigh the food, don't eyeball it. UK feeding guidelines tend to over-feed in real life. Use a kitchen scale for the first fortnight.
- Don't switch during anything else stressful — new baby, boarding, fireworks weekend. Stress alone causes loose stools, and you won't be able to tell which is which.
- Same brand, different recipe is still a transition. Swapping chicken for salmon in the same range counts. Protein source is one of the biggest microbiome triggers.
- Wet to dry (or vice versa) is a bigger jump than brand-to-brand. Lean toward the 14-day version below.
- Check the poo every day. A quick scan against the UK guide to the Bristol Stool Scale for dogs takes three seconds and tells you whether to push on or hold the ratio.
The 14-day transition (sensitive stomachs, allergies, prescription diets)
Some dogs need a gentler curve. Use the 14-day version if any of the following apply: your dog has a known sensitive stomach or IBD-type history, they've had food allergies confirmed or suspected, you're moving onto a hydrolysed or prescription diet, they've recently had a course of antibiotics, or they're a puppy under six months.
| Days | Old food | New food |
|---|---|---|
| 1–3 | 90% | 10% |
| 4–6 | 75% | 25% |
| 7–9 | 50% | 50% |
| 10–12 | 25% | 75% |
| 13–14 | 10% | 90% |
| 15+ | 0% | 100% |
If you've been told to do a true elimination diet or food trial — typically 8 weeks of a single novel-protein or hydrolysed food and nothing else — your vet's instructions trump any internet plan. The whole point is the strict exclusion, not the transition curve.
Life-stage transitions: puppy, adult, senior
The same gradual rules apply, but the timing of the change matters.
Puppy → adult. Battersea and Blue Cross both recommend keeping puppies on a labelled puppy or junior food until around their first birthday — and longer (18 to 24 months) for large or giant breeds, whose bones and joints are still finishing. When you switch, use the standard 7-day plan; don't be surprised if poos are slightly looser for a week, since puppy food has a higher fat and protein density than adult food.
Adult → senior. There's no fixed "you must switch on day X." Most UK vets suggest considering a senior diet around age 7 for medium breeds, age 5 for giant breeds, and age 9–10 for small breeds. Real triggers are slowing down on walks, weight gain on the same portions, dental issues, or a vet rec tied to a chronic condition. Use the 14-day curve if any of those apply.
Adult → fresh / raw / cold-pressed. Moving from standard dry kibble to fresh-cooked (Butternut Box, Pure), raw (Bella & Duke, Nutriment), or cold-pressed shifts macros, water content and digestibility all at once. Use the 14-day version, and expect poos to look different — often smaller, sometimes softer for a few days — because the dog is digesting more of what they eat.
If you don't yet know what you're switching to, our UK Dog Food Directory scores 40+ brands on ingredient quality, transparency, value and ethics.
What to do if it goes wrong
The single most useful instruction in any transition guide is this: if your dog gets loose poos, pause — don't push through.
Step back to the previous ratio for two extra days, let the gut settle, then carry on at half-speed. Most owners ruin their transition because they read "day 7 should be 100% new food" as a deadline rather than a guideline.
If the loose poos don't settle inside 48 hours, or if your dog is also off their food, here's the sensible playbook:
- Pause the switch. Go back to 100% old food for 2–3 days until stools are firm again.
- Try a 24-hour bland diet of plain boiled chicken (no skin, no salt) and plain white rice in roughly equal parts, in small frequent meals. Blue Cross has a useful UK guide if you've not done one before.
- Consider a daily probiotic. Probiotics help re-establish the gut bacteria that get rattled by a food change. The basics are covered in our guide on probiotics vs prebiotics for dogs — and our daily powder, Super Everyday, is built specifically for this kind of UK gut-support use case (probiotic-led, plus a few digestive co-factors).
- Restart slower. Once the poos are firm again, go back to mixing — but use the 14-day curve, not the 7-day one, and watch closely.
Some breeds notoriously need this longer plan. French Bulldogs, English Bulldogs, German Shepherds, and many Cocker Spaniels have above-average rates of food sensitivity in UK primary-care data. If yours is one of them, defaulting to the 14-day version isn't being cautious — it's being realistic.
When to call your vet
Most transition wobbles settle on their own. These are the signs that mean stop the home plan and ring your practice:
- Diarrhoea that lasts more than 48 hours, or contains visible blood or black, tarry stool
- Vomiting that happens more than once or twice in a day, or any vomiting plus diarrhoea together
- A puppy under 6 months with any diarrhoea — they dehydrate fast
- A senior dog (especially one with heart, kidney or diabetic conditions) with any diarrhoea
- Refusal to eat for more than 24 hours
- Lethargy, a painful tummy, or a sudden drop in energy
- Pale gums, a hunched posture, or a swollen abdomen — get to an emergency vet today
Our UK guide to dog diarrhoea emergencies covers the red-flag signs in more detail, with the thresholds NHS-equivalent UK pet emergency services use.
The general rule: when in doubt, call. UK vet practices would much rather field a "I think it's fine but…" phone call than see your dog three days later, dehydrated.
FAQ
How long should it take to switch dog food? For most healthy adult dogs, 7 days is enough. For puppies, seniors, sensitive stomachs, allergies, or prescription diets, 14 days is safer. Some dogs need closer to 3 weeks. Slower is rarely wrong; faster often is.
Can I switch dog food cold turkey? Sometimes — for example, if a vet has told you to start a prescription diet immediately, or if a recall has made the old food unsafe. In those cases, expect 2–5 days of looser poos and a slightly fussier eater. For any non-urgent switch, gradual is better.
Why is my dog getting diarrhoea after switching food? The most common reason is moving too fast — the gut microbiome hasn't had time to adjust to the new ingredients. Less commonly it's a true intolerance to a specific protein or grain in the new food. Pause, reset, and either restart slower or talk to your vet about which ingredient may be triggering it.
Should I give a probiotic when switching? It's optional but reasonable. The evidence is decent that probiotics shorten food-change-related loose stools in dogs. If you're already giving one, keep it going. If you're not, a daily probiotic powder during the swap and for a fortnight after is a low-risk add-on.
My dog won't eat the new food — what now? First check the obvious: is the new food more or less palatable than the old? Is your dog actually hungry, or have they been getting treats and chews on top? If they genuinely refuse, drop the new-food share back down (so it's mostly old food with a sprinkle of new), warm wet food gently to release the smell, and rebuild from there. Don't starve a fussy dog into eating — that often backfires into a longer fussy phase.
Wrapping up
Switching dog food is a small project for most UK dogs — a week of careful mixing, a daily glance at the poo bag, and a willingness to pause if the gut needs more time. The dogs that struggle are usually the ones rushed through it or switched during a stressful week.
Take it slowly, watch what comes out the other end, and give a probiotic if you have one to hand. If anything looks off for more than 48 hours, that's the moment to ring your own vet — they know your dog's history, and a five-minute phone call beats two days of internet guessing.
