Tomatoes contain a substance known as tomatotine, which is an alkaloid related to solanine. As tomatoes ripen, the tomatotine is metabolized and its presence is lessened in the fruit. Therefore, ripe tomatoes are less likely to be problematic to dogs than those that are not ripe. Some of the signs that a dog has been poisoned due to eating too many tomatoes or non ripe ones are: drooling, vomiting, seizures and diarrhea. While dogs may love tomatoes, it is really best to avoid giving it to them, just to be on the safe side.