I always recommend that you take your pet to your veterinarian if you notice any unusual symptoms. In this case, it is specifically difficult to suggest a diagnosis without a thorough examination. There are two common reasons for mouth sores in cats – viruses and irritants.

A virus called calici affects the mouth, eyes and sometimes lungs of a cat and nearly always results in mouth sores. In normal circumstances there would be multiple sores spread across the tongue and lips, and would be accompanied by other symptoms such as nose or eye discharge and loss of appetite. Calici infections must be treated early as they can lead to complications such as pneumonia. The other possible viral cause is the feline herpes virus, but this also usually results in eye or nose discharge.

The other possibility is that your cat has come in contact with an irritant, which can include toxic plants, toads, cleaning fluids and anything else that can cause irritation. Does your cat have access to parts of your house or garden that he did not previously? Have you started using a new household cleaner? Do you have any new plants in your house or garden? If you answered ‘Yes’ to any of these questions it is possible that your cat is displaying a reaction to an irritant. Your vet will be able to suggest further possibilities.