Personality disorders are very difficult to treat with medication for a couple of reasons, Victoria. The first reason is that people suffering from personality disorders (and I’m speaking specifically to the cluster B disorders; my knowledge of the other two groups is minimal) are not likely to seek treatment because , by the nature of the beast, they don’t think they are disordered.
Second reason: personality disorders are most often a result of trauma, less than chemical imbalances, the way other mental disorders present.
As a matter of fact, those who are successfully treated often suffer from a coexisting biological mental illness such as bipolar disorder or clinical depression. The improvement can often be attributed to the successful treatment of the underlying chemical imbalance causing that illness.
That said, if someone with a personality disorder seeks help, consistent adherence to a successful medication regime can improve symptoms in some people, if they want to change. And good for them! I’ve heard some success stories, but I’ve never actually met one who voluntarily sought help. Here is the information sheet on BPD from the Mayo Clinic. :)