This research study is being carried out to determine the possibility that a new medication causes allergic reactions. This new medication contains two active ingredients (diclofenac and menthol) together in a gel and is being developed to treat mild to moderate pain and inflammation, such as acute sport injuries, and sprains and strains. Approximately 240 participants will have the medication (gel) applied to the skin on a small area of the upper back for approximately 48-hour or 72-hour periods over the course of six weeks. Participation in this research study is voluntary, requiring participants to visit the study clinic on 17 occasions for about 45 minutes each visit. During the visits, doctors or trained staff members will examine the amount of redness, swelling, and other signs of irritation (if any) of the skin where the medication (gel) was applied.