The main purpose of this research study is to determine whether the drug Levetiracetam (Keppra™) is effective in the prevention of panic attacks. The drug Levetiracetam (Keppra™) has been approved for the treatment of seizures by the U.S. Federal Food and Drug Administration (FDA) and is available by prescription. Levetiracetam has not been approved by the FDA for the treatment of panic disorder.