A small dog should be belly-up after eating a handful M&M's, at least according to conventional wisdom. But watching "Moose," a friend's five-pound Chihuahua, race around a living room after his sweet snack makes one wonder: Is chocolate truly poisonous to dogs?

Dogs and humans have similar tastes. Like us, they seek out sweets and have no problem indulging. But unlike humans, our canine companions experience dangerous effects from eating chocolate—it can poison them and in some cases is lethal. The hazard, however, is probably overblown, says Tim Hackett, a veterinarian at Colorado State University. Chocolate's danger to dogs depends on its quantity and quality. Large dogs can usually handle a small amount of chocolate whereas the same helping could cause problems for Moose and his pint-size kin.

Chocolate is processed from the bitter seeds of the cacao tree, which contain a family of compounds known as methylxanthines. This class of substances includes caffeine and the related chemical theobromine. Both molecules bind to receptors on the surfaces of cells and block the natural compounds that normally attach there. Low doses of methylxanthines can lead to vomiting or diarrhea in dogs, and euphoria in humans. Chocolate contains a significant amount of theobromine and smaller amounts of caffeine. If a large quantity of theobromine or caffeine is ingested, some dogs will experience muscle tremors or even seizures. These chemical constituents of chocolate can cause a dog's heart to race up to twice its normal rate, and some dogs may run around as if "they drank a gallon of espresso," according to Hackett. Moose, it seems, was on a "theobromine high."

Dogs are capable of handling some chocolate, but it depends on the animal's weight and the type of chocolate it eats. Unsweetened baking chocolate contains more than six times as much theobromine as milk chocolate, although amounts vary between cocoa beans as well as different brands of chocolate. Less than four ounces of milk chocolate is potentially lethal for Moose and other small dogs, according to the ASPCA Animal Control Poison Center.

Around every confection-centered holiday—Valentine's Day, Easter and Christmas—at least three or four dogs are hospitalized overnight in the animal medical center at Colorado State. But in 16 years as an emergency and critical care veterinarian, Hackett has seen just one dog die from chocolate poisoning, and he suspects it may have had an underlying disease that made it more vulnerable to chocolate's heart-racing effect.

Dogs that eat a small amount of chocolate should be able to filter the methylxanthines through their body and avoid veterinary treatment. But more acutely poisoned dogs are generally treated by inducing vomiting and administering activated charcoal to absorb any methylxanthines remaining in the gut or that may be circulating through the dog's digestive system.

Ultimately, Moose survived his cocoa snack. But no matter how you bake it, wrap it or melt it, chocolate and Moose don't mix.