Q. Is there a connection (genetic or otherwise) between the bipolar disorder and BPD? Is it common for people with the BPD to have bipolar disorder as well? If so, what is the connection?

If a person has both of these disorders, how do you treat them medically?

 A. A great question, and one with no easy answer. There is an old theory in medicine that most of us follow (or should follow) called parsimony of diagnosis. The idea is that you have one disease with many symptoms. BPD and Bipolar disorder overlap a great deal. Neither one is a diagnosis. Each is a behavior. Just like you can have anxiety with depression, you can have BPD and bipolar disorder symptoms co-occurring. 

There have been a number of papers on this topic in scholarly journals over the past five years. I use parsimony of diagnosis to treat. Usually, I would pick an antidepressant to treat the BPD as I view it as more clinically significant than the bipolar portion. I feel in the vast majority of cases, 
what we are calling bipolar (usually bipolar II = BPII) is actually just the impulsivity seen with BPD. That said, I am not always right. The available controlled data suggests that the SRIs, SNRIs, Serzone, and Remeron do not induce mania any more than placebo. The Hopkins and Univ of Penn groups have also shown SRIs to be effective in treating BPIIs.

When using SRIs, SNRIs or Serzone, you titrate the dosage until carbohydrate craving resolves, and that is usually enough to work.