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Tomatoes (plant and fruit) contain tomatine, an alkaloid related to solanine. As the fruit ripens, the tomatine is metabolized. Therefore, ripe tomatoes are less likely to be problematic for animals. Clinical signs of 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. (This information comes from veterinarians, and the ASPCA Animal Poison Control Center.) (All parts of the plant except the tomato itself are poisonous to humans, although some people are sensitive to the ripe fruit also.)

Tomatoes also contain atropine, which can cause dilated pupils, tremors, and heart arrhythmias. The highest concentration of atropine is found in the leaves and stems of tomato plants, with less in unripe (green) tomatoes, and even less in ripe (red) tomatoes.