Diclofenac potassium is a nonsteroidal anti-inflammatory drug (NSAID) used to relieve mild to moderate pain and swelling from various conditions such as headache, dental pain, menstrual cramps, and muscle aches. Some brands of this medication may also reduce pain, swelling, and joint stiffness from arthritis. It works by blocking your body’s production of certain natural substances that cause inflammation. This effect helps decrease swelling, pain, or fever.