Pocket BotanistThis is one of the most searched plant questions, and the answer is a clear yes with an important qualifier. Monstera (Monstera deliciosa) is on the ASPCA's list of plants toxic to cats and dogs. But the mechanism of that toxicity is what makes it, in practice, a plant that hurts rather than one that kills. Knowing how it works tells you how worried to be.
Why it is toxic
Monstera, like most of the aroid family (pothos, philodendron, peace lily, dieffenbachia), packs its tissues with raphides: microscopic, needle-shaped crystals of calcium oxalate. When a cat bites a leaf, the crystals fire into the tongue, gums and throat. The effect is immediate and sharp: intense burning, drooling, pawing at the face, gagging, and often a bit of vomiting. The plant's whole strategy is to make the first bite the last one.
The crystals are in every part of the plant: leaves, stems, aerial roots and the unripe fruit. The ripe fruit, which the plant is named for (deliciosa), is edible for humans, but a houseplant almost never produces it.
What the symptoms look like
Within minutes: drooling, pawing or rubbing at the mouth, shaking the head, and a cat that suddenly wants nothing to do with the plant. Within an hour: possible vomiting, a swollen tongue or lips, and reluctance to eat or drink because the mouth hurts. What you will not usually see: lethargy, collapse, or anything systemic. The crystals are not absorbed into the body in any meaningful amount.
A bitten leaf with a corner missing, and a cat looking sorry for itself, is the classic picture. The discomfort typically eases within a few hours and is gone within a day or two.
What to do
Take the plant away and rinse the cat's mouth gently with water if it will tolerate it. Offer water, or a little milk or plain yoghurt, which binds the crystals and soothes the burning. Watch for two things that need a vet the same day: swelling of the mouth or throat that makes breathing noisy or swallowing hard, and vomiting that does not stop. Otherwise, a vet visit is reassurance rather than necessity, but call if you are unsure; poison lines and vets are used to this exact question.
How serious is it, really?
Rarely. Calcium oxalate plants are painful but self-limiting: the pain stops the eating before a dangerous dose is reached, and the damage is local. Serious cases are almost always about airway swelling in a small cat, or a cat that repeatedly chews despite the pain, which is unusual. The truly dangerous houseplants for cats are different ones: lilies (Lilium and Hemerocallis), which cause kidney failure from a mouthful of pollen, and sago palm. Those belong nowhere near a cat.
Keeping a monstera and a cat
Most cats ignore monstera after one experience, if they try it at all. The risks are kittens, bored indoor cats, and grazers who like to chew any leaf. Height helps: a monstera on a plant stand or in a room the cat does not use. Bitter sprays made for pets work on the lower leaves. A pot of cat grass nearby gives a grazer something better. And if a cat proves determined, the plant goes to a friend and a calathea, spider plant or Boston fern takes its place; all three are on the ASPCA's non-toxic list.
The same answer for the cousins
Everything said here applies to pothos, philodendron, peace lily, ZZ plant, alocasia, dieffenbachia and anthurium, which share the same crystals. Dieffenbachia is the worst of the group for swelling. The answer to "is my plant toxic" is one of the things Pocket Botanist shows on every identification, with a severity level, so it is worth checking a new plant before it comes home.
Check before you buy
Pocket Botanist shows toxicity for cats, dogs and people on every plant it identifies, with a severity level, so a photo in the shop tells you whether a plant can come home. Free on iPhone and Android.