Cigarette Smoke: Can cause cancer.
Cigarettes: Ingestion of nicotine could be fatal.
Antifreeze: Deadly. Just 1tsp could kill. More on Antifreeze
Rat/mouse/roach/ant baits: Deadly
Fertilizers, herbicides, insecticides
All Lilies: Deadly even in miniscule amounts. More on Lilies.
Toxic Plants: List of toxic plants.
Never give a cat drugs meant for humans or dogs without your vet's direction. Human medicine, like ibuprofen, aspirin, acetaminophen or topical creams are drastically stronger than cat medicine to the point of being lethal to your cat if given inappropriately. One extra strength acetominophen tablet (500mg) can kill a 7 pound cat; only half of a 200mg naproxen tablet could cause stomach ulcers in cats.
Chocolate: Vomiting, diarrhea, seizures
Milk: Cats are lactose-intolerant. Vomiting, diarrhea.
Bones (ie chicken bones): Deadly -intestinal perforation
Tinsel: Deadly -intestinal blockage or perforation.
Scotch tape: Deadly -intestinal blockage
Strings, yarn, ribbons: Deadly -intestinal blockage
Certain flea products on the market can kill cats or cause seizures
Many flea products that are found at pet stores and grocery stores have been known to kill a number of kittens and cats each year. The active ingredient permethrin, which is also found in some flea collars, flea shampoos & sprays, could also cause seizures in certain cats. Never use these products on kittens younger than 6 weeks old.
To be safe, use only reputable flea products that have been tested to be safe on cats. We recommend that you only use Advantage, Revolution and Frontline (Plus).
More on Flea Control and what to watch out for