(CBS) — Halloween is a great night for trick-or-treaters, but it’s also one of the busiest nights of the year for the staff of the Chicago Veterinary Emergency and Specialty Center.

WBBM’s Steve Miller reports.

You’ve probably heard that chocolate is toxic to both dogs and cats.

Dr. Anne Cohen recalls a mishap from last year involving a three-year-old puggle (a beagle and pug mix). A child left a bag of chocolate Halloween candy on the floor, and then the family went out for a couple of hours.

“They came home and found all the wrappers and piles of vomit around the house,” Cohen says.

Staff had to work aggressively to help the dog.

“That dog was hospitalized I think for two days because it was so sick. It had lots of diarrhea and vomiting that started at home. And it had an increased heart rate,” she says.

Dr. Cohen says the darker the chocolate, the worse it is for your dog or cat.

Costumes can be potentially dangerous, too.

“Especially in cats – if there’s a lot of costumes around, cats like to eat (them) – and cobwebs – decorations.  Since they’re smaller, we see obstructions that can require surgery,” Cohen says.

The Chicago Veterinary Emergency and Specialty Center treats more than 11,000 dogs and cats in its ER every year.