Dr. Drew says Britney Spears needs to focus on two key areas of her life after her DUI arrest … substance abuse and bipolar disorder.

The good doc joined us on “TMZ Live” Friday and we asked him about Britney firing her sober coaches in the weeks leading up to her DUI bust.

Dr. Drew says sober coaches aren’t the solution … he says Britney’s best bet would be to address her addictions with treatment, and that in turn would help with her bipolar diagnosis.

We broke the story … Britney had pills with her in the car when she was pulled over, and our sources say it was Adderall she got from Mexico.

Dr. Drew says an Adderall-and-booze combo is terrible for someone who is bipolar … and he explains how addiction and bipolar disorder affect each other.

The way Drew sees it … Britney’s never taken the time necessary to address some of the root issues here … and he’s hoping she makes a big change moving forward.