Butter — Food safety facts
Feed with care · Dogs · full breakdown on the linked food page
High in fat and dairy — a lick is fine, but larger amounts cause vomiting and diarrhoea, and pancreatitis risk.
Short answer
A tablespoon of butter is more than a safe lick and can cause gastrointestinal upset in dogs. Butter is high in fat and dairy, so your dog may develop vomiting, diarrhea, or even pancreatitis. Monitor him for any of those signs over the next few hours. If you notice vomiting, loose stools, abdominal pain, or loss of appetite, call your veterinarian promptly.