Why is it risky?
Most adult cats are lactose-intolerant. Kittens produce lactase (the enzyme that digests lactose) but production decreases significantly after weaning. Adult cats ingesting cow's milk lack sufficient lactase to digest its lactose, leading to osmotic diarrhea and GI discomfort. Cow's milk also has higher lactose content than cat's milk, making it particularly problematic.
Affected Systems
Digestive
Symptoms
Diarrhea (often watery, within hours), vomiting, flatulence, abdominal cramping.
What To Do
Provide fresh water. Withhold food for 12 hours if vomiting or diarrhea occurs. Vet visit if severe or prolonged.
Notes
Commercial "cat milk" products (Whiskas Cat Milk, Catisfaction) are lactose-reduced and safe, they are not equivalent to cow's milk. Plant milks (oat, almond, soy) are lactose-free but not appropriate for cats, they provide no feline nutritional benefit and contain additives. Kittens under 4 weeks needing supplemental feeding require a kitten milk replacement formula (KMR), never cow's milk.
Sources
→ Cornell Feline Health Center — vet.cornell.edu
→ VCA Animal Hospitals