EXCLUSIVE
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.