My wife is currently diagnosed with Bipolar. She has tried most bipolar medications: Paxil, Prozac, Zoloft, etc. They help some, irritate other parts of her life, and generally fail to work in the long term.
She clearly exhibits black/white thinking and crazy-making behaviors. She also cycles incredibly quickly... too quickly for bipolar in my (non-professional opinion). I can be a god one moment, garbage the next, and back to wonderful later that evening.
A good indication that her psychiatrist doesn't actually believe she is bipolar, is by prescribing Prozac. Prozac is the preferred staple drug for Borderline Personality Disorder. Prozac is also the worst nightmare for pwBipolar Disorder - if prescribed alone (without a mood stabilizer) it will induce mania faster than an alcoholic binge...
Bipolar disorder is usually treated with mood stabilizers such as Lithium, Depakote or a combination drugs such as Zyprexa, along with an anti-psychotic or tranquilizer such as Xanax.
From what I have read, Borderlines seem to do OK on mood stabilizers - but I have read that Zyprexa has poor results - and Xanax seems to be a borderlines Kryptonite...I have read several accounts of Xanax having horrible results with BPD. My ex was on Xanax and her rages increased 10 fold while she was on it - same with her dissociation...
Psychiatrists prefer to "officially" diagnose a patient as bipolar rather than borderline for three main reasons.
#1 No drama with the insurance company. Bipolar is considered highly treatable - and usually fully supported by insurance companies.
#2 Many psychiatrists feel that if they diagnose as borderline - the patient will be "shunned" by future therapist/psychiatrists. They consider it almost "blacklisting" the patient. I had a therapist admit to me that she almost always diagnoses bipolar rather than borderline - because if she diagnoses borderline, the person won't get the help they need. In her words "Any therapy is better than NO therapy" and "Treatment for the two are basically the same."
This really pissed me off, but I guess if it kinda makes sense. Most people have multiple therapists/psychiatrists before they recover, and having a rapsheet of being borderline, might be the difference between getting help and getting discharged... like I said, most therapists I know either A: Refuse to treat borderlines (saying they aren't qualified) or B: will only accept 2-3 borderlines at a time. My uncle, who specializes in treating sexually abused women and children, says he can only "handle" two borderlines at a time, that they are that taxing and emotionally draining...
#3 Many psychiatrists aren't familiar enough to properly distinguish between Bipolar Disorder and Borderline Personality Disorder. Unfortunately, this seems to be the #1 problem. Bipolar disorder and Borderline Personality Disorder have a lot of the same characteristics (Grandiose/Magical Thinking, Impulsiveness, Hyper-sexuality, Recklessness on the "high" side, Extreme Depression, Anxiety, Panic attacks, Loss of interest/appetite on the Low Side), The root of the problem, however, is very different.
It takes a keen eye, and someone familiar with BPD to see what is exactly triggering the "mood swings." Bipolar disorder is a more Mania/Depression chemical cycle, a lot of the times, with no "triggers" or the triggers are very predictable, like sleep disruption, alcohol abuse, moving to a different time zone, etc etc etc - all of which are mostly "physical changes."
Borderline Personality Disorder - as with all PDs - seems to be triggers associated with interpersonal problems. Also, the "cycles" tend to last minutes/days/hours compared to weeks/months as with Bipolar. With borderline, the pwBPD can literally split a person within seconds, and also go from elated, to horribly depressed in the same time. I watched this happen numerous times.
This subject is near and dear to me. Having successfully managed my Bipolar II disorder , and also witnessing my exBPDgf, I can tell you that the illnesses are very different from one another. My struggles seemed to be more internal than external. Sure I was a royal pain in the ass to those around me - especially my parents. They had to bail me out of some tight spots... Gambling Addictions, Grandiose thinking, months where I would literally sleep for 18 hours a day, etc etc etc... but I never did take it out on them, or "split" them like what is so common for a borderline. My support system was easily accepted by me, and I knew I had a problem (although convincing me in a manic episode was quite the challenge because I felt like GOD). Also. I was never abusive. When I was manic, I would get extremely frustrated at people not "keeping up with me" or telling me to slow down or that I was "acting crazy", but I never ever split them black nor idolized them. In short, Bipolar disorder is like being on speed. During a manic phase, everything was colorful and exciting, I felt like I could run a marathon or take a bullet... I was immortal. When the high ended, a horrible withdrawal like feeling entrapped me. I was sluggish, the most simple tasks FELT like running a marathon, no motivation whatsoever...sleep sleep sleep and sleep some more... I was that drained.
After witnessing my borderline girlfriend for three years. I never once saw a manic phase. Sure she would rotate from being extremely Narcissistic to extremely self-loathing - but this was so different from the endless energy to bed-ridden depression. Also, EVERY one of her shifts seemed to be related to some interpersonal trigger, like getting fired from a Job or me not living up to her never-ending, unobtainable demands and expectations... I do think that borderline personality disorder has a strong chemical component, but in my experience, it has a lot more to do with relating to people than bipolar disorder does.
Also - the biggest difference between a person with Borderline Personality Disorder and Bipolar disorder is that A LOT of the times, Bipolar disorder is neither on a Manic Phase nor a Depression Phase - and the person is completely normal, can hold a job just fine, and is every bit as functioning as everyone else.
pwBorderlinePD don't seem to have these long durations of calm. Their lives seem to be a never-ending cycle of dysregulation. After a while, Its a horribly predictable pattern observed by those who are close to them.
Hope this helps.