Apples: All the non-meat parts of an apple -- the stem, leaves, and seeds -- contain cyanide, which is poisonous to animals and humans.
Avocado: Avocadoes contain persin, a toxic fatty-acid derivative that can cause gastrointestinal and respiratory distress, fluid around the heart, and even death. All species -- domesticated animals, cattle, even fish -- are susceptible, so keep the guac well out of reach of your pets.
Baby food containing onion or garlic: Baby food is often recommended for ill felines; Layla Morgan Wilde, cat behavior guru and founder of the Annex Cat Rescue, notes that it's "excellent for cats that have lost their appetite, but check the ingredient labels" first to make sure no onions lurk within.
Bread dough: Cindy Wenger, animal communicator, comments that "a little bit of bread dough can cause a big problem." Why? "A dog’s stomach creates the perfect warm environment to allow bread dough to do what it does best, and that’s rise," Wenger says. "Bread dough can quickly expand in a dog or cat’s stomach, causing it to distend beyond its capacity, cutting off its blood supply." On top of that, fermenting yeast can produce ethanol; once that's absorbed into the bloodstream, your pet may appear uncoordinated and disoriented. (Drunk, in other words. Not good.)
Chocolate: Large amounts cause stomach cramping and vomiting in dogs and cats. (Keep in mind too that, for a cat or small dog, a couple of mini Special Dark bars is a large amount relative to their size.)
Grapes/raisins: It's unclear how many grapes or raisins your pet would need to eat to cause kidney failure -- some sources think it could take as few as four -- but why risk it?
Mushrooms: All kinds -- not just the sketchy-looking ones in your back yard -- are poisonous to dogs.
Nuts: Macadamias and walnuts contain a toxin that affects the digestive and nervous systems of dogs, and could cause seizures.
Onions: In raw or cooked form, onions -- and their cousins, like chives and leeks -- are toxic to cats and dogs. They contain thiosulphate, which can cause vomiting, diarrhea, and shortness of breath inpets. What's worse, says pet expert Steven May of The Daily Growl, "Typically the symptoms won't show up for a day or two." May recommends taking your pet to the vet right away if you think she's eaten onions; better safe than sorry.
Sugar-free gum and mints: Sugar-free snacks and candy sometimes contain Xylitol, an artificial sweetener that's the enemy of your dog's liver.
Shine Pets: Keep track of these toxins and more with a handy infographic
Aloe: A wonderful topical treatment for humans, it's bad for cats and dogs.
Baby's breath: Also poisonous to cats and dogs. Keep bouquets out of pets' reach, or just pull this "filler flower" altogether before putting flowers in a vase.
Bulbs: Including tulips, daffodils, and hyacinths.
Chamomile: Toxic to dogs, cats, and horses.
Grass: "But my dog/cat eats grass all the time! It's what dogs/cats do!" And usually it's fine -- unless, says Wilde, "it's sprayed with pesticides." Natural grass is okay, she says. But if you don't know what the lawn guy put on the grass, don't let your pets nibble it.
Hyacinths: Not just the bulbs; the rest of the plant is poisonous as well.
Lilies: Bruce Silverman, VMD of Chicago, IL deems lilies "probably the most common natural toxin I see ingested by cats." Lilies "are toxic to a cat's kidneys after a cat licks or chews on any part of the plant or flower," Silverman says, and the cat will need IV fluids and other professional care "to try to get the kidneys back into healthy condition."
Poinsettias: Now that the holidays are over, poinsettias pose less of a danger, but some folks do replant them outdoors.
OTHER OUTDOOR AGENTS
Insects: Often harmless, but Dr. Silverman relates a funny story about dogs and cicadas: "A few years ago half the dogs in the Chicago metro area went crazy scarfing down cicadas during their 13-year-cycle. Between the diarrhea and vomiting, and the twisted ankles from all the dogs jumping into the air to catch the cicadas mid-flight, the veterinary community had its hands full." The occasional moth shouldn't be a problem, but if your pet is snacking on a pile of bugs -- or you live in an area with poisonous spiders -- keep an eye on any bug snacking.
Yahoo! News: Amazing animal migrations
Rock salt: De-icing salt can cause burning and cracking to paws. If it gets stuck between your pet's toes and he licks his feet to work it loose, it could irritate his stomach. If your pets go outdoors (and cats generally shouldn't), add a quick paw rinse to your wintertime post-walk routine, and check the animal's feet to make sure uncomfortable boluses of salt or dirt haven't gotten trapped.
COFFEE, BOOZE, AND CIGARETTES
Alcohol: "Some people think it's cute or funny for a pet to drink, i.e. a beer, not realizing alcohol is toxic to both cats and dogs," Wilde says.
Caffeine: Could cause collapse and seizures, among other symptoms, in pets.
Nicotine: Smoking kills -- secondhand smoke is bad for pets, too -- and nicotine in any form, whether cigarettes, patches, or gum, can cause heart and respiratory failure in pets.
Acetaminophen, ibuprofen, prescription drugs, and medications intended for use by humans should never be given to pets. Topical preparations for humans -- sunscreen; bug repellent; rubbing alcohol, e.g. -- should also be kept well out of their reach.
And drugs and medicines that are intended for your furry friends should be administered as directed. Do not borrow prescriptions from friends, or freelance the dosage; do as your vet advises, and if you aren't sure how to give a medication, call and ask.
SYMPTOMS TO WATCH FOR
Panting or shallow breathing
If your pet doesn't display these symptoms, but you saw the cat nibbling a daffodil or the dog is behaving oddly after digging in the trash, don't take chances. Call your vet, an emergency-care clinic, or an animal poison-control hotline (here's the ASPCA's) right away.