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Why does my adult cat get runny diarrhea every time she drinks a few laps of cow's milk?

Cats · Food safety

Last reviewed: 2026-09-05 · Reference ranges: ASPCA, Pet Poison Helpline & Merck Veterinary Manual

Short answer

Most adult cats are lactose intolerant because they lose the enzyme needed to digest lactose after kittenhood. The undigested sugar draws water into the intestines, leading to gas, bloating, and diarrhea. This is a common intolerance, not an allergy, so the reaction is usually limited to digestive upset. If your cat enjoys dairy, a small amount of plain, unsweetened yogurt or lactose-free milk is often better tolerated. Otherwise, it's safest to stick to fresh water and avoid giving milk altogether.

Milk & dairy — Food safety facts

Feed with care · Cats · full breakdown on the linked food page

Many adult pets are lactose-intolerant — milk often causes diarrhoea. Small amounts of plain yogurt are usually better tolerated.

Dogs and cats carry the same rating (Feed with care), so the notes below hold for either pet.

Milk & dairy 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

Added sugar brings empty calories, and over months that means weight gain and dental disease — both far harder to undo than to avoid.

Most adult pets make very little lactase, so dairy tends to produce gas, bloating and diarrhoea instead of usable nutrition.

The full verdict, portion guidance and serving notes are collected on the Milk & dairy food facts page.

Open the full food page: Milk & dairy food facts →