https://www.akc.org/expert-advice/nutrition/can-dogs-eat-marshmallows/ (excerpt below)
Marshmallows are one of those treats that can be hard to resist — and it’s nearly impossible to eat just one. But, can dogs eat marshmallows? The answer is no. Though not all marshmallows are toxic to dogs, they certainly aren’t good for your canine companion. Made from sugar, corn syrup, gelatin, vanilla extract, and coated with either cornstarch or confectioners’ sugar, marshmallows contain very little, if any, nutritional value or health benefits.
Dr. Carly Fox, a staff doctor at New York City’s Animal Medical Center, says if the marshmallow has xylitol (an artificial sugar) as an ingredient, it is absolutely toxic to your dog and can be extremely harmful, even if ingested in small quantities. “Xylitol can cause dangerously low blood sugar, leading to seizures and even death if the dog is not treated properly,” says Dr. Fox. “It has also been shown to be toxic to the liver, even days after ingestion.”
