Bam Margera's Brother Says He's On The Run With Girlfriend and a Child, Using Meth

Bam Margera is on the run from the police with his girlfriend and her 8-year-old child, and to make matters worse, he's developed an addiction to meth ... at least according to his brother, Jess Margera.

The former "Jackass" star's sibling is making the alarming allegations on Twitter, claiming Bam is in a really bad place right now.

Jess says Bam is traveling by car with his girlfriend and her daughter, and he says he "can't even imagine the environment in that car night now," worrying about the kid's safety and adding ... "she doesn't deserve this unnecessary trauma."

Bam's brother claims he's picked up a meth habit recently and is now "unrecognizable." Jess says he doesn't know what Bam's capable of when he's using meth and says it's been "scary" and "heartbreaking" to see Bam "hallucinating."

TMZ broke the story ... Bam's got a warrant out for his arrest in Pennsylvania, where State Police say Bam got into an altercation with his family and then made a getaway into the woods.