A patient complains of feeling nervous or fearful. These feelings and behaviors suggest that the patient has an anxiety disorder, and the doctor prescribes whatever drug will most probably work for an anxiety disorder. However, there's no conclusive way to tell that this patient definitely has an anxiety disorder. Even if the doctor did get the diagnosis correct, there's a great deal of variation regarding which drug class (for example, anti-anxiety drugs versus antidepressants) a particular individual will respond to and which drug within a class (for example, Prozac versus Zoloft) will work best. If the drug doesn't work, the doctor will try the next one on the list and so on, thus delaying treatment success and complicating the process with the mix-and-match type of treatment.