Pets are a surprisingly huge part of the Halloween experience these days.

Case in point: The National Retail Federation says 16 percent of people who celebrate the holiday will dress their pet in a costume.

But don’t think that means the pets can take part in the trick-or-treating. In fact, the American Veterinary Medical Association has an important warning for pet owners: Don’t feed your pets candy—especially if it contains chocolate or the sugar substitute xylitol.

In a recent interview with Mental Floss, AVMA President Mike Topper noted that chocolate in particular has significant dangers for both dogs and cats—though cats, being pickier eaters, are much less likely to be interested in chocolate.

“They just can’t break it down as fast as humans, and so therefore, when they consume it, it can cause illness,” he told the website.

That slow metabolization of the chemical theobromine, which humans can easily consume but dogs can’t, can create an array of negative effects for pets, ranging from an upset stomach to heart failure or even death. The differences can be significant between breeds.

“A Great Dane who eats two Hershey’s kisses may not have the same [reaction] that a miniature Chihuahua that eats four Hershey’s kisses has,” he added.

Topper says that if your dog eats chocolate, you should probably make your way to the vet, quickly.

Beyond candy and chocolate, AVMA warns that you should keep your pets away from candles, jack-o’-lanterns, and glow sticks. A microchip might also be a good idea in case your pet darts out when trick-or-treaters visit your house.

And as for that budding pet dress-up trend, AVMA says it’s OK to put your pet in a costume generally, but it recommends buying a costume that’s specially designed for pets rather than creating your own.

“If you plan to put a costume on your pet, make sure they will tolerate it, it fits properly and is comfortable, doesn’t have any pieces that can easily be chewed off, and doesn’t interfere with your pet’s sight, hearing, breathing, opening its mouth, or moving,” the association emphasizes on its website.

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