One hundred and twenty (120) adult outpatients with bipolar I, II, not otherwise specified, or cyclothymic disorder and current cocaine dependence will be enrolled. After obtaining informed consent baseline assessment measures will be administered including the Structured Clinical Interview for Diagnostic Statistical Manual-IV Axis I Disorders. Drug use will be assessed using the timeline-followback method to quantify days and amount of drug use, urine drug screens will also be obtained and craving will be assessed with the Cocaine Craving Questionnaire. Mood symptoms will be quantified at each weekly visit with the Hamilton Rating Scale for Depression (17-item version), Quick Inventory of Depressive Symptomatology-SR (QIDS-SR), and Young Mania Rating Scale (YMRS). Impulsivity will be assessed at weeks 0, 5 and 10 with the Barratt Impulsiveness Scale (BIS, Barratt et al 1983). Cognition will be assessed at weeks 0, 5, and 10 with the Rey Auditory Verbal Learning Test (RAVLT) and STROOP color-word task. The Addiction Severity Index (ASI) will be administered at baseline and week 10. The Psychobiology of Recovery in Depression-III Somatic Symptom Scale (PRD-III)will be administered every 2 weeks to track side effects. A study psychiatrist will assess participant-reported side effects weekly. Women of childbearing age will be given a test to rule out pregnancy. Subjects will be randomized and Lamotrigine therapy or identical appearing placebo add-on therapy in a double- blind fashion will be initiated at 25 mg/day and increased to 200 mg/day using a slow upward titration over 5 weeks (as outlined by Calabrese et al 2000 and following the package insert) to minimize risk of side effects such as rash. After that time additional increases in 100 mg/day increments to a maximum of 400 mg/day can be made if the medication is well tolerated and HRSD scores have decreased by ≤ 40% from baseline or Cocaine Craving Questionaire (CCQ) scores have decreased ≤ 25% from baseline or participants continue to use cocaine in past week based on either self-report or urine drug screen results. Subjects will be assessed weekly for mood and drug use/craving and every four weeks for cognition over 10 weeks. All of the assessments may be provided in Spanish, if needed. Additionally, a Spanish-speaking research assistant and study psychiatrist will be available at all times.