The famed doctor dropped by "TMZ Live" on Monday to explain why a 2nd conservatorship is pretty unlikely in Brit's case. He says they're pretty tough to come by in California, and Britney -- who was previously diagnosed as bipolar -- would have to exhibit some extreme behavior before a court would even consider creating a new conservatorship.

They're more difficult to be granted in the years since Britney was first placed in one.

For instance, he says Britney would need to be a harm to herself or others, or lose the ability to take care of herself ... before she could be placed on an involuntary psychiatric hold.

Dr. Drew noted, "No one can really do anything. It's the horrible thing. You can try to do an intervention, maybe bring a group of people. A show of force sometimes is helpful. And then, urge her to get further care."

Based on her history, though, he's certain Brit would not be open to an intervention -- "She was in a long-term dual-diagnosis program, did beautifully [and] now claims that was a program that was abusive to her and was holding her prisoner -- which was nonsense."

As multiple sources previously told TMZ ... Britney's mental health has been in a tailspin since her conservatorship ended in November 2021. The singer is allegedly not taking her prescribed meds ... instead drinking and using various drugs.

Britney and her boyfriend, Paul Soliz, got into a physical altercation at the Chateau Marmont hotel earlier this month ... resulting in paramedics being called to the scene.