FDA Approved Medication to Reduce Binge Eating and/or Purging

Inclusion Criteria: * Must meet Diagnostic and Statistical Manual -Edition 5 criteria for bulimia or binge eating disorder, which has not responded to prior treatment * Men and women between the ages of 18-60 * Must be at least normal weight (i.e. body mass index at least 21.0 or greater) * If female of child-bearing potential, the participant must have an initial negative pregnancy test and be taking adequate birth control * must be medically stable with no new diagnoses (medical, surgical, or psychiatric) within the past 6 months * Must live close enough or be within commute distance of study site (Stanford, CA) to be able to comply with study visits, treatment plans, and blood draws and other possible assessments (i.e. EKG) Exclusion Criteria: * Any patient with bipolar disease or schizophrenia or any patient taking a mood stabilizer or antipsychotic medication within the past 3 months * Any patient with current or past history of anorexia nervosa (i.e., with a Body mass index of \< 17.5). * Any patient with prior (within 30 days) use of Over The Counter (OTC) weight-reducing agents, herbal preparations * Any patient who has been prescribed a medication for weight loss within the past 3 months * Any psychological weight-loss interventions (i.e., Weight Watchers, Jenny Craig) in which patient has lost weight over the past month (unless willing to discontinue). * Any patient currently in therapy with a psychostimulant or who has used a psychostimulant within the past 6 months * Any patient with known sensitivity to phentermine or topiramate * Any patient who has had a change in thyroid, psychiatric, or blood pressure medications within the past one month * Any patient who is taking a potassium-wasting diuretic * Any patient who is on a carbonic anhydrase inhibitor such as zonisamide acetazolamide, or dichlorphenamide * Any diabetic who is taking insulin or an insulin secretagogue * Any patient with liver enzymes at baseline greater than three times the upper limit of normal * Any patient with a baseline potassium of less than 3.0 millequivalent/Liter. Patients with baseline potassium levels between 3.0-3.5 miliEquivalents/Liter will be encouraged to ingest high potassium foods such as bananas, oranges, or tomato juice daily during the study * Any patient with abnormal baseline thyroid test or with a Thyroid Stimulating Hormone (TSH) greater than 1.5 times the upper limit of normal * Any patient who is unwilling or expresses uncertainty about being able to refrain from daily alcohol use (even if at moderate levels) or illegal drugs while taking the study medication. * Any adult with recent history of suspected alcohol or substance abuse or substance dependence * Any patient who regularly uses marijuana (including medical marijuana) or illegal drugs * Any patient who has a history of addiction to a stimulant * Any patient who expresses current suicidal ideation * Any patient with a history of nephrolithiasis * Any patient who is pregnant or who is planning to become pregnant during the study period. * Any patient who is currently participating in any other clinical study that involves an active treatment * Any patient who is unable to identify a primary care physician * Any patient with a history of cardiovascular disease (i.e. recent history of Myocardial Infarction, stroke, shortness of breath, chest pain) that could increase vulnerability to the sympathomimetic effects of a stimulant like drug. If clinical suspicion is high, baseline EKG will be obtained and anyone with abnormal findings will be excluded