Depression is not a single illness; it's likely to be several different illnesses that fall under a single umbrella diagnosis.

Please don't make the mistake of thinking bi-polar is in anyway similar to depression. They are very different illnesses with different treatments.

And, again, as we learn more about bi-polar we may find that it's a bunch of different illnesses that get given the umbrella diagnosis of bi-polar.

Your example of ADHD being treated with the same meds is just a function of our lack of knowledge of ADHD. So we might not always use the same meds for treatment.

In the future we'll have genetic testing to determine what meds will be useful or which meds are unlikely to work.