Justin Bieber -- Abusing Drugs, Including Sizzurp

Justin Bieber is using drugs with alarming regularity ... and it's making him so erratic his people have been urging him to go to rehab ... sources closely connected with the singer tell TMZ.

Multiple credible, connected sources tell us ... Bieber has been using sizzurp regularly. It's typically codeine and promethazine mixed with Sprite and a Jolly Rancher. Bieber and his crew refer to it as "lean." It's a very dangerous narcotic that can trigger seizures, and even worse.

But that's not the only drug Bieber is using. We're told he's been taking lots of prescription pills, including Xanax, in addition to lots of alcohol and weed.

Our sources insist he does NOT do cocaine and has never done it in his life.