My cat’s upper left lip is swollen and recently grew larger

From the photo, there is an abnormal lump inside the mouth and lip with redness, ulceration, and a tendency to grow, so prompt veterinary testing and treatment are advised. Likely causes include oral lesions from tumors or foreign-object injuries; check the cat’s mouth carefully.

Early on you can use an oral spray with anti-inflammatory medication to stop spread, but if there is no improvement, have a biopsy to determine the nature of any tumor. Calicivirus can also cause stomatitis with drooling, bad breath, and black crusts at the mouth, needing targeted treatment. Allergic reactions can cause eosinophilic granuloma with lip swelling and drooling, which also requires desensitization therapy at a vet.