The "Britney Spears" trial has officially begun, and right out the gate Sam Lutfi's lawyer told the jury ... "She liked to use amphetamines. Most everything that went wrong was because of this drug."
Sam's lawyer, Joseph Schleimer, admitted in opening statements ... Sam used Adderall (an amphetamine) for ADD, but he was adamant, Sam never gave her the drug.
And Schleimer claimed, Sam repeatedly told Britney she needed to get off drugs, to no avail. Schleimer says Sam wanted to bring a new team in -- he called it "the varsity team" -- to help save her.
And Schleimer says in an attempt to clean Brit's house of drugs, he brought in a drug-sniffing dog and found a baggie of crystal meth.
Schleimer also said Britney's dad Jamie Spears attacked Sam while Sam was working for Britney, punching Sam and knocking the wind out of him.
As for Sam's managerial role with Britney -- Schleimer says Britney approached HIM to be her manager, offering 15% of her total earnings ... $800,000 a month when she wasn't on tour, and MILLIONS when she was.
Schleimer also gives Sam credit for hooking Britney up with her fiance Jason Trawick -- claiming he introduced the two at Jason's Halloween party in 2007.