Viruses and bacteria are common causes of acute airway symptoms in cats, especially if the eyes are also running. But sneezing that becomes long-lasting or affects one side can have other causes, such as chronic inflammation, foreign objects, dental problems or more unusual changes in the nasal cavity.
Notice whether the discharge is clear, pus-like or bloody and whether it comes from one or both nostrils. Appetite matters too, as a blocked-up cat may eat less when its sense of smell is affected. With recurring or long-lasting problems, the vet may need to go further with imaging or tests.
Book a vet if the sneezing persists, becomes severe or comes with fever, reduced appetite, or a pus-like or bloody nasal discharge. Seek emergency care if your cat has difficulty breathing or breathes with its mouth open.