
Signs of poisoning can include excessive drooling, vomiting, diarrhea, seizures, lethargy, nosebleeds and loss of appetite. Your cat may only exhibit one or two of these symptoms and cats are very good at hiding pain so it's always important to take note of any sudden changes in your cat's behaviour.
Mistletoe and Holly - These are moderately to severely toxic - if you think your cat has ingested either of these then contact your vet.
Lilies, Daffodils, Amaryllis and other plants in the daffodil family - these are severly toxic and can cause kidney failure and death if ingested. Daffodil bulbs are particularly toxic. A lot of these plants start going around as gifts in "bulb kit" form at this time of year so keep these out of reach of your cats.
Poinsettias - These are the Christmas pot plant of choice with their lovely red and green foliage but they're mildly toxic to kitties if ingested. They're not highly toxic but they can cause irritation, diarrhea and vomiting.
Christmas Trees - These aren't really toxic but the sap can cause vomiting and drooling if ingested and the needles aren't easily digested so can cause irritation in the stomach and bowel if eaten in quantity.
Other things include wrapping materials such as ribbon, tinsel and lametta. I came home to several pools of vomit last night as Clive managed to chew the ribbon off some of the presents under the tree and swallow it. The only reason I knew was because the vomit was laced with lots of ribbon... very festive vomit but not ideal so I've now removed the gifts with ribbon from under the tree. Clive also eats lametta so I can't use it on my tree anymore. Not all cats are quite as idiotic as him but do watch out for this if your cat likes to swallow things it shouldn't.
Small batteries - cats are far less likely than dogs to eat these but apparently there's a big increase in battery related vet visits at this time of year due to all the toys readily accessible to pets.
Christmas lights - if your cat is a wire chewer then keep an eye out for them having a go at your christmas lights!
Artificial snow - this is toxic to cats so ensure they don't take a fancy to it and start licking it off your windows

Chocolate, onions, nuts, blue cheese, fruit cakes, mince pies, christmas pudding - these all contain things that are harmful to your cat in varying degrees - some are more dangerous than others so best to keep consumption of human food to a minimum with your kitties. If you give them Turkey then make sure there aren't any bones in there as cooked bones in particular are very likely to splinter and get caught in your cat's mouth or throat.
Enjoy the festive season and enjoy your furries! We should start a GC thread with christmas kitty photos
