The green portions of tomato plants contain significant quantities of the toxic alkaloid tomatine -- bad for dogs -- but ripe, red tomatoes contain very little. Ripe red tomatoes are essentially harmless for dogs. However, do not feed Fido ketchup or pasta sauce, as they contain several other ingredients that are bad for him, such as onions, chives, and even garlic in large doses.
Tomatoes are members of the deadly nightshade family (Solanaceae), which includes tobacco, eggplant, potatoes and tomatillos. Tomatoes and their relatives possess several dangerous alkaloids, although they are not present in all parts of the plant. While the plant's ripe fruits, which are technically a type of berry, are safe for dogs, all other portions of the plant -- leaves, stems and vines -- are toxic. Dogs who eat these portions of the plant may exhibit drooling, diarrhea, vomiting, decreased heart rate, tremors, seizures, irregular heartbeat, lethargy, dilated pupils or paralysis. Take your dog to a veterinary hospital immediately upon observing these symptoms.
PetMD recommends that pet owners refrain from offering their pets tomatoes. By contrast, Debbye Turner, resident veterinarian on the CBS television program "The Saturday Early Show," asserts that ripe, red tomatoes are harmless for dogs, although they may cause digestive upset to those with sensitive stomachs. The Pet Poison Hotline largely agrees with Turner's claim, stating that tomatoes are "barely poisonous and generally pretty safe to feed dogs." The Merck Veterinary Manual includes pages dedicated to six toxic foods -- avocado, bread dough, chocolate, macadamia nuts, raisins and the artificial sweetener xylitol -- but makes no mention of tomatoes.
Tomato plants contain a chemical called alpha-tomatine concentrated in their green portions. Ripe tomatoes possess small amounts of the chemical. Tomatine can be toxic to your pet's heart, but a dog would need to eat "massive" quantities of tomatoes to ingest enough of the toxin to cause problems, according to Ottowa-based veterinarian Marie Haynes, who operates AskAVetQuestion.com. Haynes says tomatoes are harmless for cats as well, despite popular sentiment to the contrary. Part of the reason they are not harmful, Haynes explains, is that the intestine does not absorb tomatine very well. Accordingly, Haynes considers tomatoes safe for canines and felines to eat. The amount of tomatine in a tomato decreases over time as the fruit matures and metabolizes the chemical. Accordingly, the riper tomato, the safer it is for your dog.
Even if tomatoes are safe for your dog to eat, the garlic, chives and onions in your favorite pasta should give you pause. These related vegetables can be toxic for your pup in large doses, although the level at which toxicity occurs in each is not well understood. Garlic powder is an ingredient in many dog treats, and some authorities suggest that small quantities of garlic are beneficial. However, it is difficult to know how much garlic is in a given tomato sauce. Onions are not safe; small quantities may be dangerous. Ingestion of these foods can lead to anemia and permanent kidney damage. Contact your veterinarian if your dog eats any of these foods.