What causes a red sore on my dog's lip and when should I see a vet?

Hello, thank you for your question about Bear.

Based on Bear's relatively young age and the photo that you have provided I would be suspicious that Bear is experiencing a mild bacterial or viral infection in this area. Puppies are often more susceptible to viruses such as the papilloma virus more than adult dogs, this virus typically forms cauliflower like growths in/around the mouth, but certain forms can be more mild and not lead to these signs. It is possible that Bear has a bacterial infection, these are usually fairly benign and self limiting.

Another possibility is that Bear is experiencing sensitivity or an allergy to something in his environment leading to this lesion. Some dogs are very sensitive to certain types of food bowls or toys, and can develop these lesion secondary to this.

I would recommend having Bear evaluated by your veterinarian so that they can examine the lesion and Bear's mouth. They can discuss the need for additional diagnostics such as an impression smear or gentle sampling of the area. Depending on these findings they can then make recommendations for necessary treatments such as antibiotics.

Hope this helps, good luck!