Lamotrigine is a novel anticonvulsant. Clearly it is useful for treating epilepsy, in terms of reducing seizure frequency in difficult cases, with a better side-effect profile than many other anti-seizure medications.
It has been used in psychiatry for over 10 years, mainly to treat bipolar depression. Initial studies were quite encouraging. More recent studies have been a bit more mixed.
Here are a few recent references:
this was a large study of using lamotrigine as an augmentation to treat unipolar depression. It did not show that lamotrigine was useful overall. But it appeared much more effective in a subgroup of patients with much more severe depression. Average doses were about 200 mg/d, maximum 400 mg.
this study showed improvements in affective lability with lamotrigine vs. placebo, in patients with borderline personality, using a flexible dose (average about 100 mg/day).
http://www.ncbi.nlm.nih.gov/pubmed/19200421 200 mg/d lamotrigine helpful for bipolar depression as add-on with lithium x 8 wks
no difference in depression scores with lamotrigine vs placebo as add on for bipolar patients
http://www.ncbi.nlm.nih.gov/pubmed/22351381 adding lamotrigine 100 mg to an SSRI improved symptoms (about 30% reduction in YBOCS score) in OCD patients, compared to placebo, after 16 weeks.
In conclusion, I think lamotrigine is in the "why not give it a try" category for a variety of problems. It might possibly help for treating depression in bipolar patients, or even as an augmentation in unipolar depression. It might be useful in OCD patients, and in borderline personality. It may be one of those agents which helps some individuals very well, even though the average effect for a group of similarly afflicted individuals may be unremarkable. Another possibility is that it could help with certain symptom domains, such as obsessionality, ruminativeness, etc. which could occur more prominently in some but not all patients with mood or personality disorders.
I think the big advantage of lamotrigine is that it is quite well-tolerated, with a benign side-effect profile, except for a small risk of a dangerous rash and a few other rare serious problems, which just need to be watched for carefully.