Butter is very high in fat and can cause a cat’s intestinal tract and pancreas to become inflamed, leading to gastroenteritis and pancreatitis.
Some cats may develop vomiting and diarrhea, which can be mild, but in severe cases, it can cause more significant issues such as abdominal pain, decreased appetite, and weight loss.
Suggestions
Most cats can tolerate a small lick or two of butter, but it's best to avoid giving them larger amounts.
If you want to share a snack with your cat, lean meats such as chicken, turkey, beef, tuna, haddock, and shrimp are much safer options.
Note: even if labeled as safe, any food can potentially cause allergies. When introducing a new food, please feed a small amount and observe your cat for any allergic reactions.