How much molasses is too much for a medium-sized dog, and what happens if he eats a whole tablespoon?

For a medium dog, even a teaspoon of molasses is a significant amount of sugar, and a whole tablespoon is too much. Molasses is very high in sugar and potassium, which isn't toxic in a single small lick but can cause problems in larger quantities. Eating a tablespoon could lead to an upset stomach, vomiting, diarrhea, or a temporary spike in blood sugar. It's not an emergency for a healthy dog, but it's best avoided. If he seems unusually thirsty, lethargic, or has digestive issues, give your vet a call.