For pets, Halloween is a time to bark at decorations, be scolded for trying to eat pumpkin, and suffer the indignity that goes along with being forced into a costume.

There’s actually one other consequence to the season, and it’s significantly more serious than getting dressed as an Ewok. Halloween candy is everywhere, and it can really do some damage to furry members of the household.

According to the Pet Poison Helpline, calls from concerned pet owners increase 12 percent during the week of Halloween. That’s because there are a number of opportunities for dogs and cats to get into potentially toxic sources of food—bowls full of candies, bags from trick or treating, and the occasional bite-sized treat that gets dropped on the floor.

One of the worst offenders is chocolate, which is basically a primary ingredient in Halloween candy and contains theobromine, a chemical that dogs and cats can’t tolerate. The darker the chocolate, the more theobromine it contains. One square of baking chocolate could be as toxic as 23 chocolate drops.

While a pet’s reaction can vary depending on how much they consume—20 mg of theobromine per pound of bodyweight is going to be a problem—you’ll likely see vomiting, diarrhea, panting, thirst, and possibly even seizures.

Even if the candy contains little to no chocolate, pets will still have a problem. Sugar and fat can lead to pancreatitis, the symptoms of which (vomiting, diarrhea, reduced appetite) may not develop for a few days. Sugar-free might even be worse, as xylitol, a common sugar substitute, is toxic to dogs even in small amounts.

Boxes of raisins or grapes can be trouble, too. Even a small amount of either can cause kidney failure.

If you suspect your pet has ingested something harmful, it’s best to contact your veterinarian or the Pet Poison Helpline at (855) 764-7661. But the best approach is prevention. Bags or bowls of candy should always be out of reach from pets; kids should be taught never to share their treats; giving them ample exercise, particularly around Halloween, might also help prevent them from getting too curious.