Why does my cat have a lip sore? Is it a rodent ulcer or allergy?
This appears to be what is commonly referred to as a 'rodent ulcer'. The most common cause for these to pop up in cats is an allergy, typically to something in the food.
Sometimes these will resolve in their own, however sometimes they can become infected, long lasting, and problematic. If they become very large and painful it can discourage the cat from wanting to eat.
If this ulcer has been present for more than 2 weeks it is probably not going to go away on its own. At that time I would recommend taking you cat to the vet for treatment. Nathaniel may need antibiotics and an immunsuppresive drug like steroids or Atopica.
If the lesions keep coming back he may need to be on an immunosuppressive long term and/or a hypoallergenic diet.
I hope this helps. Thanks for using Petco Pet Education Center, formerly Petcoach. Best of luck.
Updated on September 23rd, 2025