People with fibromyalgia report generalized body pain (pain all over), increased sensitivity to painful stimulation, chronic tiredness or low energy, sleep problems, and other physical and functional problems. The exact cause of the disorder is poorly understood, and treatment can be difficult. The degree to which duloxetine is helpful for people with fibromyalgia varies greatly. For some people, it is very helpful for managing fibromyalgia symptoms. For others, people may not notice any benefit. Yet for some, it is a little helpful and the effect is noticeable only when people forget to take the medicine. The purpose of this study is to collect data to better understand the relationship among gene types that control those enzymes, blood concentrations of duloxetine, and how it helps the symptoms.