Diclofenac sodium is a nonsteroidal antiinflammatory drug (NSAID) that has been used in 120 countries since its introduction in Japan in 1974. It is currently the eighth largest-selling drug and the most frequently used NSAID in the world. Diclofenac, a phenylacetic acid derivative, is a potent inhibitor of cyclooxygenase enzyme activity, and may also interact with the lipoxygenase enzyme pathway, and with the release and reuptake of arachidonic acid. Diclofenac is almost completely absorbed, highly protein-bound, penetrates well into synovial fluid, and is extensively metabolized. Comparative studies have shown that diclofenac is at least equivalent in efficacy to aspirin and other NSAID when used for the treatment of rheumatic diseases such as rheumatoid arthritis, osteoarthritis, and ankylosing spondylitis. Diclofenac also possesses potent analgesic properties. Clinical trials suggest that diclofenac has a favorable side-effect profile, excellent patient tolerability, and a lower patient dropout rate when compared with aspirin and other NSAID.