To begin with, before bringing a new cat home, you need to wander around your house and assess safety as you would for a toddler. There seems to be a consensus that you should not leave out anything you would not leave out and handy to a 2 year old. Keep in mind that there is not really an “out of reach” for a cat without putting it in a cabinet. With some cats I have known, that isn’t even enough, but it should suffice in most cases.
Keep all medication away from cats, especially OTC painkillers. Both aspirin and ibuprofen are toxic to them, and Tylenol contains acetaminophen, another cat-specific toxin.
Antifreeze is toxic to cats. Then again, antifreeze is toxic to most things that are not a car, so you’ve probably tucked that away somewhere safe already. Similar statement applies to cleaning products. Again, unless you lean towards green and non-chemical cleaning, they are fairly toxic.
Remove potentially dangerous houseplants – peonies, lillies, hyacinths, mistletoe, and evergreens.
Keep your toilet lids down. Kittens can drown trying to drink the water, and if you treat the water in the tank, adult cats can poison themselves doing the same.
If you keep cut flowers around, make sure all elements of the arrangements are non-toxic. A lot of ornamental plants are poisonous to both cats and people, but the cats may actually chew on them. (Lillies are a good example.)
Keep the flue to your fireplace closed. Cats are more than able to climb up a chimney, and between the soot, and the getting out (if you have an indoor cat) this can cause all sorts of complications.
Take care with ironing boards. They were not made stable enough to stay standing after being assaulted by a leaping cat.
Take precautions with electrical cords. Some cats will chew them.
Do not leave unattended candles. Cats are drawn to warmth, and could knock them over, or singe themselves, or worse.
Be careful with twine, string, ribbon, yarn, dental floss, etc…. Cats tongues are barbed, and long lengths of things can be involuntarily swallowed, as the design of both tongue and throat will just keep drawing the material down.
