Justin Bieber arrives at The 40th American Music Awards in L.A. Credit: Jon Kopaloff/FilmMagic

First marijuana, and now . . . sizzurp? TMZ is reporting that Justin Bieber was recently photographed partying with rapper pals Lil Za and Lil Twist -- with a bottle of codeine cough syrup (known colloquially as "sizzurp") nearby. The website also reports that the 18-year-old "Beauty and the Beat" singer was snapped drinking an unknown substance out of a double cup. Rappers like Lil' Wayne have openly spoken about downing sizzurp -- which gives a serious narcotic high -- out of double cups.

TMZ has yet to publish the sizzurp photos in question, and notes that sources close to Bieber deny that he abuses the powerful drug. On Jan. 4, the site posted photos of the singer smoking what appeared to be marijuana (with Lil Za and Lil Twist) in a California hotel room.

"Everyday growing and learning. Trying to be better. U get knocked down, u get up," Bieber tweeted Jan. 5, acknowledging the controversy. "I see all of u. I hear all of u. I never want to let any of you down. I love u."

A source told Us Weekly at the time that his team "isn't happy about those photos. They are grilling security about who could have taken them."