Tomato
Plant.

What it does to a cat.

Yes — the tomato plant is toxic to cats. But the tomato fruit, when ripe, is not. The ASPCA lists Lycopersicon species as toxic to cats, dogs, and horses. The toxic principle is solanine, the same glycoalkaloid that runs through other members of the Solanaceae family (potato, eggplant, nightshade). It is concentrated in the leaves, stems, and unripe green fruit. The ASPCA explicitly notes that the ripe fruit is non-toxic.

This is one of the few plant pages where the answer is not "keep it away from your cat." It is "keep your cat away from the vine, but the fruit you grew is fine to share."

Why the green parts and the ripe fruit are different

Solanine is a defensive alkaloid the plant produces in tissue that needs protection from herbivores — leaves, stems, and unripe fruit. As a tomato ripens and turns red, the solanine breaks down. In a fully ripe red tomato, solanine concentrations are far below the threshold for toxicity in a cat. In a green tomato or a tomato leaf, they are not.

A related compound called tomatine appears in the unripe fruit specifically. Tomatine and solanine produce similar clinical signs: hypersalivation, vomiting, drowsiness, and at higher doses CNS depression with slow heart rate and dilated pupils.

What it actually looks like

Most reported cases in cats involve leaf or stem chewing — a curious cat in a kitchen-garden patio or a windowsill pot. The first sign is drooling within an hour or two. Vomiting and diarrhea follow. Most cases resolve within 24 to 48 hours with supportive care.

Large exposures — particularly green-fruit ingestion — can produce weakness, unsteadiness, slow pulse, and dilated pupils. These warrant a vet visit even if the cat seems otherwise alert.

What to do if your cat ate tomato plant

For a leaf or green-fruit ingestion, monitor at home and offer water. Most cats are uncomfortable but recover. Call a vet if vomiting persists, if the cat is unsteady or unusually drowsy, or if you saw a substantial amount eaten. ASPCA Animal Poison Control is available 24/7 at (888) 426-4435.

Cat-safe kitchen-garden substitutes

For a windowsill herb pot you can grow alongside cooking, basil, rosemary, and thyme are all ASPCA non-toxic and live in the same conditions as tomato. (One caveat: the "Spanish thyme" sold as Plectranthus amboinicus is ASPCA toxic and is covered on the thyme page — buy English thyme, Thymus vulgaris, if you want a safe pot.)

For cats that love the texture of greenery itself, catnip and oat or wheat cat-grass are non-toxic redirection plants.