Research by Frank Wilhelm and Walton Roth published in the journal "Anxiety Disorders" found that use of alprazolam during flight increased anxiety. Panic attacks also increased with the use of alprazolam. The bottom line, according to the researchers is, "Results indicate that alprazolam increases physiological activation under acute stress conditions and hinders therapeutic effects of exposure in flying phobia."

You can read more on the research study at this link.