The short answer: It is not recommended.
The long answer: Marshmallows are pillowy treats that are made of water, gelatin, and sugar (usually sucrose or corn syrup). Though not exactly toxic to dogs, these confections are not recommended for your dog’s daily consumption because of their high sugar level and low nutritional value.
What to do if your dog accidentally eats marshmallows: If your dog managed to munch on one or two pieces of marshmallows as you’re preparing them in the kitchen, it shouldn’t be a problem as long as the marshmallows are not sugar free or coated in chocolate.
Sugar-free marshmallows may contain xylitol, an artificial sugar substitute that’s highly lethal for dogs. The smallest amount can do damage to your dog and cause low blood sugar, weakness, lack of coordination, depression, tremors, seizures, liver failure, coma, and/or death. Similarly, chocolate-covered marshmallows present a grave problem because of the theobromine found in it. This component causes vomiting, diarrhea, tremors, spasms, seizures, and/or heart attack in dogs.
If your dog consumes regular marshmallows, observe him for any changes. If your dog eats marshmallows that contain xylitol or chocolate, take him to your veterinarian immediately.
In summary: Though it’s tempting to treat your dog to some marshmallows while you’re preparing a mug of hot chocolate or a batch of s’mores, you’re better off giving him healthy snacks like fresh fruits, vegetables, or protein-packed dog treats. Marshmallows are packed with sugar and artificial flavourings which will not benefit your dog. Regularly feeding him marshmallows could lead to weight gain or diabetes, and make him more likely to develop cancer, fungal, and yeast infections.