Inositol hexaphosphate (IP6, also called inositol hexakisphosphate, and phytic acid) is a naturally occurring phosphorylated derivative of myo-inositol. Myo-inositol has shown preliminary evidence of efficacy in controlling mood symptoms, and good tolerability in bipolar disorder in some studies, but failed to establish efficacy in subsequent meta-analyses. In the investigators proposed work, the investigators plan to orally administer the calcium/magnesium salt of IP6 (2,000-3,000 mg daily in two divided doses) to paid research subjects with a diagnosis of bipolar disorder who are in a depressed state, and who have failed an adequate course of treatment with lithium monotherapy. The investigators hypothesis is that IP6 may be similar to myo-inositol in terms of relieving depression, but more potent and effective. Our aim is conduct a preliminary pilot study in 30 subjects (15 treated with IP6, 15 treated with lamotrigine, an active comparator) to assess the efficacy and tolerability of IP6 as an adjunctive treatment to lithium, the mood stabilizer most commonly used to treat bipolar disorder.

Inclusion Criteria:

• must have bipolar disorder
• must be currently depressed
• must have failed adequate trial of lithium monotherapy
• must have shown partial response of depression to lithium

Exclusion Criteria:

• diagnoses of schizophrenia, major depression, or other psychotic disorder
• currently pregnant
• unstable medical condition
• active drug or alcohol dependence
• concurrent use of antidepressant or mood stabilizer other than lithium
• active suicidal or homicidal ideation
• past adverse reaction to lamotrigine or current skin rash (lamotrigine arm only)
• history of dietary malabsorption or nutritional deficiency (IP6 arm only)