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New food, new problems — or so it seems when a sudden switch brings vomiting, diarrhoea or a hunger strike. The gut needs time to adapt to new ingredients, and cats in particular distrust unfamiliar food. The fix is simple: transition gradually over 7–10 days. This guide gives you the schedule, the warning signs, and the special cases.
Short Answer: Mix the new food in gradually over 7–10 days: 75% old / 25% new for days 1–2, 50/50 for days 3–4, 25/75 for days 5–6, then 100% new from day 7. Stretch it to 10–14 days for sensitive stomachs. If stools loosen, pause at the current ratio for a few extra days. Puppies, kittens and pets on prescription diets need vet guidance before any switch.
Last reviewed: 3 November 2026 · The Paw Pilot editorial team
On this page
- Why sudden switches upset stomachs
- The 7–10 day transition schedule
- For sensitive stomachs: the 14-day plan
- Switching between food types
- When to slow down or stop
- Special cases
- Frequently asked questions
Why sudden switches upset stomachs
The gut microbiome — the community of bacteria that digests food — adapts to what it is fed regularly. A sudden change in protein sources, fibre levels or fat content overwhelms it, and the result is soft stools, gas, vomiting or refusal to eat. Cats add a behavioural layer: neophobia (suspicion of new food) is normal feline behaviour, and a cat that stops eating entirely risks fatty liver disease. Gradual transitions let both the microbes and the mind adjust.
The 7–10 day transition schedule
This is the standard schedule recommended by veterinary nutritionists (including WSAVA guidance):
| Days | Old food | New food |
|---|---|---|
| 1–2 | 75% | 25% |
| 3–4 | 50% | 50% |
| 5–6 | 25% | 75% |
| 7+ | 0% | 100% |
Mix the foods in the same bowl at each meal so the pet cannot eat around the new food. Keep mealtimes, location and routine unchanged — familiarity helps acceptance.
Try the Daily Food Portion Calculator
New food, new calorie density — recalculate the right daily portion for the new food’s kcal per cup or can.
For sensitive stomachs: the 14-day plan
Pets with a history of digestive upset, seniors, and cats that are suspicious of change do better on a slower schedule:
| Days | Old food | New food |
|---|---|---|
| 1–3 | 90% | 10% |
| 4–6 | 75% | 25% |
| 7–9 | 50% | 50% |
| 10–12 | 25% | 75% |
| 13–14 | 0% | 100% |
Switching between food types
Dry to wet (or reverse). Transition as normal, but expect portion sizes to look very different — wet food is mostly water, so the volume will be much larger. Recalculate portions from the new food’s calorie content.
Puppy/kitten to adult food. Small breeds can switch at about 9–12 months, large and giant breeds at 12–18 months or later, kittens at about 12 months. Ask your vet for timing — switching too early short-changes growth.
Between flavours of the same brand. A shorter 3–5 day transition is usually fine, since the formulas are similar.
To or from raw/home-cooked. Transition slowly and discuss nutritional balance with your vet — see the FAQ in How Much Should I Feed My Dog?.
When to slow down or stop
Slow down (stay at the current ratio for 3–4 extra days) if you see soft stools, extra gas, mild vomiting or reduced enthusiasm for meals.
Stop and call your vet if you see bloody diarrhoea, repeated vomiting, lethargy, abdominal pain, or refusal to eat for more than 24 hours (cats) — especially in puppies, kittens, seniors or pets with health conditions. These can signal that the new food disagrees fundamentally, or an unrelated illness.
Special cases
Puppies and kittens. Young animals dehydrate faster and have less reserve — transition carefully and call your vet early if digestive signs appear.
Prescription/therapeutic diets. Never switch onto or off a prescription diet without your vet’s direction; the transition plan is part of the treatment.
Cats that won’t eat the new food. Warm it slightly to release aroma, try a topper of the old food’s “juice”, and be patient — but never let a cat go without eating for more than 24–48 hours. An overweight cat that stops eating risks hepatic lipidosis (fatty liver disease).
Food discontinuation. If the old food is unavailable, transition as best you can with whatever remains; if none remains, start at 50/50 with small, frequent meals and go slowly.
Frequently asked questions
Can I just switch cold turkey?
Some pets tolerate it, but it is not worth the gamble — digestive upset is common and entirely avoidable with a week’s transition.
My pet picks out the old food and leaves the new. What now?
Mix thoroughly, slightly moisten dry mixes so flavours blend, and avoid topping up with extra old food. Hunger is a great sauce — a healthy pet will not starve itself over preference (but watch cats closely, per the warning above).
How long until stools are normal?
Mild softening during transition is normal and should resolve within a few days of reaching 100% new food. Persistent loose stools beyond a week on the new food warrant a vet call.
Do I need to transition between flavours of the same brand?
A short 3–5 day blend is enough for most pets, since the base formula is similar — but don’t skip it entirely.
Should portions change with the new food?
Almost certainly yes. Calorie density varies widely between foods, so recalculate from the new label’s kcal per cup or can (see How to Read a Pet Food Label) rather than feeding the same volume.
Related guides
- How Old Is My Dog in Human Years?
- How Much Should I Feed My Dog?
- How Much Should I Feed My Cat?
- Is My Dog Overweight?
- Is My Cat Overweight?
- How Much Does a Puppy Cost the First Year?
- How Much Does a Kitten Cost the First Year?
- How Many Treats Can My Dog Have a Day?
- How to Read a Pet Food Label
- How to Switch Your Pet’s Food Safely
Disclaimer: This guide is for general information only and is not a substitute for veterinary advice. If your pet has digestive disease, is on a prescription diet, or shows worrying symptoms during a food change, talk to your veterinarian.