Diclofenac is a nonsteroidal anti-inflammatory drug (NSAID). It works by reducing substances in the body that cause pain and inflammation. Diclofenac topical (for the skin) is used to treat joint pain caused by osteoarthritis. Pennsaid is for use on the knees. Voltaren Topical is for use on the hands, wrists, elbows, knees, ankles, or feet. This medicine may not be effective in treating arthritis pain elsewhere in the body. Diclofenac topical is used to treat warty overgrowths of skin (actinic keratoses) on sun-exposed areas of the body.