How to change your dog's food without upsetting their stomach

PDSA explains that most dogs eat the same food every day for most of their lives, so a sudden change can be a shock to the gut. Diarrhoea tends to develop because the gut has not had time to adjust, even when the new food is perfectly suitable.

Diarrhoea and extra wind are the most common signs. Some dogs vomit or refuse the new food, and the Royal Kennel Club notes that some take time to adapt to new tastes. Battersea advises against changing too much of a dog's routine at once.

Advice on timing varies. Blue Cross sets out a seven day switch in four steps. PDSA takes about two weeks, starting with a teaspoon of the new food in a separate bowl. The plan below uses Blue Cross's proportions at PDSA's gentler pace.

Swap food rather than adding it, so the total stays the same, and weigh it if you can. Finish on the new pack's feeding guide, since amounts vary between products. Treat each step as a minimum: dogs with sensitive stomachs, young puppies and fussy eaters may need longer.

• Days 1 to 3: the usual meal, plus a teaspoon of the new food in a separate bowl. Do not mix them yet.
• Days 4 and 5: about a quarter new food, three quarters old.
• Days 6 and 7: half new, half old.
• Days 8 to 10: about three quarters new, a quarter old.
• Days 11 to 14: phase out the old food, once your dog has eaten the mix happily for at least a week and their poo is firm.

Your dog's appetite and poo should decide the speed, not the calendar. If their poo softens or they start leaving food, go back to the last mix that suited them. Blue Cross advises stopping the new food if your dog vomits or has diarrhoea, then trying again later, more slowly.

Never force your dog to eat, and never leave them hungry until they give in. The Royal Kennel Club advises calling your vet if a dog has eaten nothing for 24 hours, or a puppy for 12 hours, because puppies need to eat regularly.

Do not keep offering something new, either: the Royal Kennel Club says this can reinforce a taste for novelty. PDSA suggests set mealtimes instead, lifting the bowl after 15 minutes if it is untouched and offering fresh food a couple of hours later. Keep toppers and treats small; Dogs Trust suggests treats make up no more than 10 per cent of the diet.

The gradual approach applies to almost every change. What differs is when you start, and whether you or your vet sets the timetable.

If your dog has a health condition or a history of tummy trouble, talk to your vet before changing their food.

• A new puppy: keep to the breeder's food and routine while they settle in, then change slowly, over a week to ten days or longer.
• A rescue dog: ask what they have been eating, and offer the same or a similar food while they settle in.
• Puppy to adult food: the right age depends on size. Blue Cross suggests around a year, or up to about 18 months for large breeds; ask your vet about timing.
• A vet-prescribed diet: follow your vet's plan for the switch, even if it differs from this one.
• The old food runs out: if you can, buy a small bag of it to finish the change properly.
• Home-prepared meals: introduce them gradually too. They must be complete and balanced, which PDSA says is extremely difficult to get right, so involve your vet before they become the main diet.

UK Pet Food explains that complete is a legal term on pet food labels: a complete food gives your dog all the nutrients they need, in the right amounts, when fed as directed. A complementary food is only part of the diet and must be fed with something else, such as mixer biscuits. Blue Cross notes that wet food in tins and pouches can be either, so read the pack.

Use the new pack's feeding guide, or your vet's amount, as a starting point, then adjust to your dog's weight and body condition, as Dogs Trust advises. In Pup Kitchen, recipes are labelled complete or complementary, the portion tool gives a starting estimate rather than a prescription, and each recipe reminds you to check with your vet before changing your dog's diet.

Keep a short record while you switch: what your dog ate at each meal, how much, whether they finished it and what their poo was like. It tells you when to move on or step back, and gives your vet facts rather than guesses.

Snootly's Daily Log lets you record each meal with the food type, amount and whether it was finished, and add a note to a poo entry. Anyone you invite to share your dog's care sees it too. Snootly Agent can help with questions, but it is AI: its answers can be wrong and it does not replace your vet.

A mild upset often settles within a few days, PDSA says, but stop the switch and call your vet if you see any of the signs below. Call sooner for a very young, elderly or unwell dog. Collapse, or repeated retching with a swollen tummy, is an emergency: contact your vet straight away, or an out of hours vet.

• Diarrhoea or soft poo lasting more than 24 hours.
• Blood or mucus in the poo, or blood in vomit.
• Repeated or continuous vomiting.
• Low energy, a painful tummy or signs of dehydration, such as sticky gums.
• An adult dog who has eaten nothing for 24 hours, or for 12 hours alongside vomiting or diarrhoea.
• A puppy who has eaten nothing for 12 hours, or any puppy with diarrhoea.
• Weight loss, or a dog who is neither eating nor drinking.