Do Medications Work in Borderline Personality Disorder?

That said, there are certain aspects of BPD that may benefit from medication (impulsivity, mood swings/affective instability, and even rejection sensitivity). 

According to both the American and European recommendations, what is the primary treatment for borderline personality disorder? 
Psychotherapy. In the USA, Dialectical Behavioral Therapy is the first line treatment for BPD. 
It is important to note the following 3 facts about medications in BPD: 

1. No medication been approved for the treatment of borderline personality disorder. So everything we consider is off-label. 
2. Almost all patients with BPD are treated w/ medication.
3. Usually treated with multiple medications across various classes. The average patient with BPD is on 3 different medications for treatment. Fifty percent are on 4 or more medications.
   

So no studies have shown a clear enough benefit to give FDA approval for BPD. Does that mean medications are not useful at all?
No. There are 2 big factors to consider: 

1. The studies of medications to treat BPD are, themselves, flawed a specific way. Namely, they exclude over 90 to 95% of patients with BPD because they exclude all patients with BPD who are also suffering from "current comorbid psychiatric diagnoses" such as PTSD, major depressive disorder, PTSD, suicidal thinking, and other important comorbid disorders that many patients with BPD often have. So when you exclude all of these patients, you are left with a subset of patients with BPD that don’t often reflect the real world. 
2. Many studies do show positive effects of various medications on certain symptoms of BPD, but they are not necessarily all “placebo controlled” studies. These studies are sometimes overlooked by larger meta-analyses that tend to pool results of many studies to then create treatment guidelines. This misses many potentially helpful treatments.
   

What are the medication classes that do show benefit in aspects of BPD? 

1. Some of the 2nd generation antipsychotics can be helpful for affective instability. Quetiapine (especially Seroquel XR) has been shown to be most helpful with affective instability and perceived interpersonal rejection.
2. Some medication classes, including mood stabilizers and alpha-2 agonists, can help the impulsivity seen in BPD. 
3. Omega 3 fatty acids have considerable benefit in BPD. 
4. Clonidine does have evidence supporting it in BPD for impulsivity and rejection sensitivity. "Orally given clonidine may be effective for treatment of acute states of aversive inner tension, dissociative symptoms, and urge to commit self-injurious behavior in female patients with borderline personality disorder." 
   https://pubmed.ncbi.nlm.nih.gov/15491247/
   

What if someone with BPD has acute symptoms of depression, or acute symptoms related to trauma/PTSD? 
Yes, those should be treated. That is what makes this discussion so complex— many patients with BPD are also suffering from other disorders that may benefit from traditional medication management, such as SSRIs for depression, prazosin for nightmares in PTSD, stimulants for ADHD, etc. 
The most important point to make about this is that borderline personality disorder (BPD) is a personality construct, so medications do not do a lot to change the underlying etiology or symptomatology in the disorder.