This is only my .02 worth, so take it for what it's worth. This is based on my experience working at a hospital years ago, and 2 friends who were in relationships (actually one still is) with a bi-polar person (later in life, not people at the hospital)

If you're starting in a relationship with a person who is legit bi-polar (diagnosed - not just assumed), run, don't walk. The swings are unnerving. The feeling of worry, of never feeling 100% safe, or concern over medication, of whether the person will just go missing or commit suicide.

These are my opinions not based on hundreds of cases, just the ones I've known in the real world.

If it's a family member, tough love with them staying on their meds and stay hopeful advances in the treatment will inch closer to understanding it. That's a long way off, since there isn't even a full consensus as to what's happening in the brain (there's a part of the brain that sends like 30% more signals than a non bi-polar brain). Most believe it's genetic since studies of twins indicate that if one identical twin has bi polar, there's an 80% chance the other twin will have it (versus like 15% in fraternals)

#1 rule if you're with someone - religiously make sure they take their meds. (lithium most likely in the mix). It dramatically drops the suicidal thoughts and regulates the mood swings for many.

Good luck with however this plays out for you.