Diclofenac (marketed as Voltaren, Voltarol, Diclon and Cataflam) is a non-steroidal anti-inflammatory drug (NSAID) used to relieve the inflammation, swelling, stiffness, and joint pain associated with rheumatoid arthritis, osteoarthritis (the most common form of arthritis), and ankylosing spondylitis (arthritis and stiffness of the spine). Diclofenac is also prescribed for immediate relief of pain and menstrual discomfort.

Diclofenac is often used to treat chronic pain associated with cancer, particular if inflammation is also present (Step I of the World Health Organisation (WHO) Scheme for treatment of chronic pain). Good results (sometimes better than those with opioids) have been seen in female breast cancer and in the pain associated with bony metastases.

Fever due to malignant lymphogranulomatosis (Hodgkin's lymphoma) often reponds to diclofenac. Treatment can be terminated as soon as the usual treatment with radiation and/or chemotherapy causes remission of fever.

Diclofenac may prevent the development of Alzheimer's disease if given daily in small doses during many years.