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The usual suspects

Short answerBefore you suspect a mysterious allergy, look at the everyday causes: a dog eating something richer, fattier or faster than his gut was ready for. Table scraps and fatty foods, scavenged finds, a sudden new chew, too many treats and dairy top the list. True food allergies exist - Merck's manual notes they're less common than airborne allergies - and they're diagnosed with a vet-guided elimination trial, not by switching brands at random.

Sources: the Merck Veterinary Manual, linked at the end.

The everyday triggers

Rich and fatty extrasSkin, fat trimmings, gravy, fried food and buttery leftovers are the classic next-day-upset foods - and heavy fatty loads can trigger more serious problems, like pancreatitis in prone dogs.

Scavenged anythingBin raids, compost, dropped picnic food, what the toddler shared. When the stool goes wrong, ask "what did he find?" before "what's wrong with his food?"

Sudden new treats and chewsA first marrow bone or a new rich chew is a dose of fat and novelty at once. Introduce treats like food: small amounts, one new thing at a time.

DairyMany adult dogs handle lactose poorly. A little plain yogurt may be fine for some; a shared ice cream is asking for tomorrow's soft stool.

SpeedThe fastest trigger of all is switching food overnight - even between two good foods.

Intolerance is not allergy

The Merck Veterinary Manual's allergy guidance draws a line worth knowing: an intolerance (this food upsets his stomach) is a digestive problem, while a true food allergy is an immune response - most often to a protein - that typically shows up as skin signs like chronic itching, sometimes with digestive upset alongside. The distinction matters because the fixes differ: intolerances are managed by avoiding the trigger, while suspected allergies are worked up properly.

The elimination trial, briefly

If your vet suspects a food allergy, the standard tool is an elimination-challenge diet: weeks of a strict novel or hydrolyzed protein diet - nothing else passing the lips, no treats, no flavored chews - followed by deliberate reintroduction to see what provokes signs. Merck's allergy guidance stresses the same two rules that make it work: absolute strictness during the trial, and patience, because the schedule runs in weeks - up to about three months - not days. Done casually, it tells you nothing; done properly, it ends the guessing.

A simpler first step

Before any of that: two weeks of boring consistency. One food, minimal plain treats, zero table food, and entries in the stool log. A surprising share of "chronic sensitive stomach" is actually a daily drizzle of rich extras nobody was counting. The log makes that visible - and if the gut stays upset on a truly boring menu, that's meaningful information for your vet too.

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