Diclofenac is in a class of drugs called non-steroidal anti-inflammatory drugs (NSAIDs). Diclofenac works by reducing hormones that cause inflammation and pain in the body. Diclofenac is used to reduce pain, inflammation and stiffness caused by many conditions, such as osteoarthritis, rheumatoid arthritis, abdominal cramps associated with menstruation, and ankylosing spondylitis. Diclofenac may also be used for purposes other than those listed in this medication guide.