A dropped campfire sweet is not a dog treat
The marshmallow that rolled off the stick and landed beside your campfire should go in the fire or the bin, not to your dog. Marshmallows are essentially whipped sugar and corn syrup: they deliver a fast sugar load, stick to teeth, and offer a camping dog nothing he needs. Their soft, expanding texture is also awkward for dogs, who tend to gulp sweets whole, and a large marshmallow can be a choking problem for a small dog snatching quickly before you can intervene. On a camping trip, where routines are loose and treats are passed hand to hand, marshmallows are a food to manage deliberately rather than share casually.
The sugar-free packet changes everything
Before anything else, read the packet your marshmallows came from. Sugar-free and low-sugar marshmallows are often sweetened with xylitol, also listed as birch sugar, and xylitol is extremely dangerous for dogs even in small amounts, causing a rapid insulin release and potentially liver injury. If the dropped marshmallow was sugar-free and your dog ate it, that is not a wait-and-see situation: contact a vet or emergency animal hospital immediately, even if he seems completely fine, because early treatment is what changes the outcome. Regular marshmallows do not carry that specific danger, but the packet check takes ten seconds and decides how seriously to treat any campsite theft.
If one plain marshmallow already vanished
A single plain marshmallow eaten by a medium or large dog will usually cause nothing worse than a sugar-fuelled half hour and possibly a loose stool later, so panic is not required. Offer water, skip any further sweet treats that evening, and keep an eye on him around the fire, because a dog rewarded by one successful grab will patrol for the next dropped sweet. For a very small dog, or a dog with diabetes or a weight problem managed under vet guidance, call your vet for advice even after a plain marshmallow, since sugar loads matter more in those cases. Repeated vomiting, wobbliness, or unusual tiredness after any sweet-eating episode deserves a same-day vet call.
Campfire habits that prevent the next grab
Toasted marshmallows add one more hazard worth knowing about: a molten marshmallow pulled straight from the flame can burn a dog's mouth, and the gooey strands cling to teeth and fur. Keep roasting sticks raised and away from the dog's settle spot, and count dropped sweets back in at the end of the evening so none are left in the grass for a dawn patrol to discover.
Keep the marshmallow bag sealed, clipped, and stored off the ground in the food box rather than open beside the chairs, and give dropped sweets a quick kick into the fire before a hopeful nose arrives. Set your dog up with his own long-lasting chew or a portion of his normal food in a puzzle toy during dessert, so the s'mores round does not leave him as the only one without a project. Brief everyone around the fire, especially children, that the dog's treat is his own biscuit, not a toasted marshmallow pulled apart for him. Recording the no-sweets rule and any accidental grab in KinsLe through Care Teams keeps sitters and fellow campers consistent, so one person's campfire soft-heartedness does not undo everyone else's care.