Whey — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Plain whey protein is not toxic, but most powders are sweetened — check for xylitol.
Dogs and cats carry the same rating (Feed with care), so the notes below hold for either pet.
Whey is not poisonous to cats, but it is easy to over-feed. Rich fat, salt or seasoning can bring on vomiting or diarrhoea within a day, and repeated large portions are a recognised trigger for pancreatitis.
If it ate some before you noticed, watch for vomiting, diarrhoea, loss of appetite, unusual tiredness or a painful, swollen abdomen over the next 24 hours. A one-off small amount in a healthy adult usually passes with water and rest; repeated exposure, a puppy or kitten, a senior, or a pet with an ongoing illness is a reason to call your vet the same day.
Why these cautions matter
Even a gram or two of xylitol can trigger a sharp drop in a dog's blood sugar, and larger exposures go on to damage the liver.
Added sugar brings empty calories, and over months that means weight gain and dental disease — both far harder to undo than to avoid.
Whey can turn up now and then, but not as a daily habit: keep the rest of the day's food unchanged and watch the litter box and how quickly they finish the next meal.
The full verdict, portion guidance and serving notes are collected on the Whey food facts page.