Zac Efron — Post-Rehab Reunion With ‘Hairspray’ Cast

1011-moby-derek-houghZac Efron returned to the L.A. nightlife scene Thursday … for the first time since news about his 2 stints rehab went public … and it was all to celebrate HALLOWEEN!!

Efron appeared in great spirits as he hit up the Griffith Park Haunted Hayride with former “Hairspray” co-stars Julianne Hough and Brittany Snow … as well as director Adam Shankman.

our sources broke the story — the 25-year old actor completed 2 rounds of treatment earlier this year to combat the cocaine problem that took over his life. 

Julianne”s brother Derek Hough snapped a pic with the group sharing hugs and smiles — and posted it online afterward saying, “Great time last night with great friends.”

You can”t stop the beat.