Diclofenac is a nonsteroidal anti-inflammatory drug (NSAID) that works by blocking the production of prostaglandins, which are chemicals that cause pain and inflammation.Additional information:It is a prescription medication.Do not take it if you are allergic to diclofenac sodium or other NSAIDs, or if you have a history of stomach ulcers, bleeding disorders, or asthma attacks triggered by NSAIDs.It can interact with other medications, so it is important to tell your doctor about all the medications you are taking before you start taking it.