Purple drank or "lean" is a name for codeine mixed with a soft drink and candy. It is a cough suppressant in small doses, but in larger doses it is quite similar to PCP(!)
It's theorized that Trayvon Martin was a user, based on facebook posts about the drug. See also this.
Trayvon: "unow a connect for codine?"
Mackenzie: "Why nigga?"
Trayvon: "To make some more."
Trayvon: "Codeine is a higher dose of dxm."
Trayvon: "You can get the same vibe off lean."
Nice in depth discussion here.
It is quite possible that Trayvon Martin – experiencing the tolerance effects caused by chronic use of DXM - potentially took a larger dose of DXM than he’d had before, or at the very least was recently indulging himself in an admitted actitity. Maybe even suffered a “psychotic break”, but at least the behavioral paranoia and psychological stress consistent with the use of long-term DXM use might have to a physical and psychological reaction to questions by George Zimmerman, and ultimately to his aggressive response.