Your dog grabs a discarded piece of gum before you can turn away. If they swallowed it, contact your veterinarian or an animal poison service immediately. Some sugar-free gums contain xylitol, also called E967, which can cause a dangerous drop in blood sugar and liver injury in dogs. You may not know the brand from a wrapper found on the pavement. Do not wait for symptoms or try to make the dog vomit unless a professional instructs you.
Tell the veterinarian when the gum was picked up, how much you think was swallowed, your dog's weight, and any packaging or ingredients you can identify. Bring a photo or the wrapper if safe to collect. The gum's other ingredients and wrapper matter too. If your dog seems weak, shaky, unsteady, or collapses, seek emergency veterinary care immediately while contacting the clinic.
Prevent the next pickup
For later walks, scan ahead and change paths when rubbish is scattered. Use a well-fitting harness and a lead you can control. Practice "leave it" with safe objects indoors, rewarding your dog for turning away, then build gradually outside. Do not use real gum as a training prop. For a dog who repeatedly swallows street items before you can intervene, ask a qualified trainer or veterinarian about a properly fitted, gradually introduced basket muzzle as one part of a safety plan.
The immediate question is not why your dog scavenged. A possible xylitol ingestion needs medical guidance first.
Sources: Dogs Trust: What foods are toxic to dogs. Dogs Trust: How to train your dog to leave it.