The "drug" he was arrested for was "codeine syrup" according to the Mobile County Sheriff's Office.
What is "codeine syrup?"
Codeine syrup is described as medicinal drug that is also used to get high. The reason Jamarcus Russell was arrested was that he did not have a prescription for its use in his possession. It's the main ingredient in what's called "Purple drank," a slang term for "a recreational drug popular in the hip hop community in the southern United States" according to Wikipedia.
Jamarcus Russell posted bail
Jamarcus Russell posted bail of $2,500 after being arrested at 4:22 EDT according to the Press-Register.