This research study is being done to assess if a combination of medications can enhance the relief of superficial pain (pain at the surface of the skin, such as sunburn pain). The investigators also want to find out if certain genes may be linked to individual differences in experienced efficacy of pain killers. The combination of medications under investigation is diclofenac and atropine. Diclofenac has been approved by the U.S. Food and Drug Administration (FDA) to treat pain. Atropine has been approved by the FDA to treat certain types of poisoning, heartbeat problems, and other diseases but atropine is not approved to treat pain. However, atropine has been used for many years in different European countries to treat painful conditions such as stomach cramps.This research study will compare diclofenac and atropine to placebo.