Cat swollen upper lip

Question
Q;My rescue domestic short hair male has had a recurring swollen lip for the past year. He has been getting Convena injection every six weeks, now he is swollen after 9 days. I have switched to Hypoallergenic Hydrolyzed food, yesterdays news litter, stainless steel feeding dishes, and all plastic eliminated. He is only 2 years old, they  want to do a biopsy . What can this be?

Answer
This sounds like something called a "rodent ulcer."  It is how many cats show allergies.  It is part of something called the eosinophilic granuloma complex.  It is usually treated with either a shot of a long acting steroid or a new oral medication called Atopica.