Zac Efron -- Friends Believe He's Into Drugs, Not Sushi

Zac Efron's friends are "worried sick" about the actor ... and none of them believe Zac drove to downtown L.A. in the middle of the night to get sushi -- they say there's no doubt he's fallen off the wagon again.

TMZ broke the story ... Zac ended up in an area of downtown L.A. that turns dangerous late at night, with transients and drug dealers out in force. Zac told cops his car ran out of gas, his "bodyguard" got attacked and when he went to help he got socked in the face.

Cops are skeptical, and it's echoed by Zac's friends. As one friend put it, "Zac's a loner. He goes out rarely and the idea of him driving for miles to go to some sushi restaurant in downtown L.A. after midnight is preposterous."

Several of Zac's friends tell us the so-called bodyguard is not a bodyguard at all. We've confirmed he's a convicted drug dealer who has been spending a lot of time with Zac recently ... despite warnings from friends.

There were a number of reports Zac went to rehab twice last year to kick his cocaine habit. But sources now tell us ... Zac NEVER went to rehab. Both times he went to a friend's house outside California and received private therapy --- but that's it.

Zac has lost friends in the last few months because they became frustrated that he was slipping off the rails again.