In dogs, xylitol sets off a big insulin release that drives blood sugar down fast.

A veterinary xylitol review puts the low-blood-sugar line at more than 0.1 gram per kilogram of body weight, and the liver-failure line at more than 0.5 gram per kilogram.

For a 10-pound dog, that first line sits under half a gram of xylitol.

Signs usually show up within 30 to 60 minutes: vomiting, weakness, stumbling, collapse or seizures. Some products delay the drop for up to 12 to 48 hours.

Grapes, raisins and currants have been linked to kidney failure in some dogs, usually within 72 hours, according to the Merck Veterinary Manual.

The leading suspect is tartaric acid. Dogs clear organic acids poorly, so it builds up in the kidneys.

ASPCA vets put that clue together after a dog got sick from homemade playdough made with cream of tartar, which is a form of tartaric acid.

Tartaric acid levels vary from grape to grape, which is why a handful leaves one dog fine and puts the next on IV fluids.

Merck lists roughly 800 mg of these compounds per ounce of dry cocoa powder, about 440 mg in unsweetened baking chocolate, 150 to 160 mg in semisweet and dark chocolate, and about 64 mg in milk chocolate.

Mild signs such as vomiting, diarrhea and heavy thirst can start at 20 mg per kilogram. Heart effects show up around 40 to 50 mg per kilogram, and seizures at 60 mg per kilogram and up.

For a 20-pound dog, that works out to about 1.2 ounces of dark chocolate for mild signs and about 3.5 ounces for seizures.

The same dog can get sick from less than half an ounce of baking chocolate.

That same 20-pound dog would need close to 3 ounces of milk chocolate to reach mild signs.

A pile of candy is also a pile of sugar and fat. Merck notes that the fat in chocolate can trigger pancreatitis in some dogs, even when the theobromine dose stays low.

A dog that eats the bar usually eats the wrapper too. Foil and plastic can block the gut.

The doorbell. A dog who reacts to the sounds dogs hate gets a full night of them. Set your dog up in a quiet back room with water and a chew during trick-or-treat hours.

The open door. The front door opens dozens of times. Check that your dog's ID tag is on and the microchip info is current.

The costumes. Strangers in masks can scare even a calm dog. If your dog wears a costume, make sure it doesn't block sight, hearing or movement.

The candles. Jack-o'-lantern candles sit right at tail height.

One-ingredient freeze-dried treats like beef liver, chicken hearts or minnows. They're small, high-value and free of sugar.

A spoonful of plain canned pumpkin. Pie filling has sugar and spices, so check the can.

The poison control numbers saved in your phone before the first knock: ASPCA Animal Poison Control at (888) 426-4435 and Pet Poison Helpline at (855) 764-7661. Both may charge a consultation fee.