Tomato Plants (unripe tomatoes or the plant part) - tomatoes contain tomatine, an alkaloid related to solanine and atropine. As the fruit ripens, the tomatine is metabolized. Therefore, ripened, red tomatoes are not likely to be harmful to a dog when eaten.
Tomato plants (the vines, stems and leaves), however, are toxic and can cause serious symptoms if eaten by a dog.
Green, unripened tomatoes can also be potentially harmful, but contain less of the toxic chemicals - tomatine and atrophine - than the plant parts and therefore might simply cause stomach upset.
Atropine is concentrated in the vines and leaves of a tomato plant and can cause dilated pupils, tremors, and heart arrhythmias. Tomatine triggers abnormalities with the nervous system, kidneys and digestive tract.
Symptoms: Tremors, seizures, heart arrhythimias. Clinical signs of tomatine poisoning include lethargy, drooling, difficulty breathing, colic, vomiting, diarrhea or constipation, widely-dilated pupils, paralysis, cardiac effects, central nervous system signs (e.g., ataxia, muscle weakness, tremors, seizures), resulting from cholinesterase inhibition, coma and death.
Action: Take your pet to the vet or emergency clinic for care.