Monteith was found dead July 13 in Vancouver from overdose of a mix of alcohol and heroin.
Efron's friends say he has been in a bad shape for quite some time and they fear he could relapse and overdose, causing death like Monteith, reports showbizspy.com.
"Zac had been on a downward spiral since 2011, he was just good at hiding it and not getting caught," said a source.
"He was partying because he was unhappy and basically just didn't give a f**k," added the source.
The 25-year-old recently posted a new photo on his Instagram account following reports about his rehab stay and cocaine problem.
"Hey guys!" he posted with the photograph.
"Just returned from an incredible trip to Peru with my dad and wanted to thank you all for your support these past few weeks...means the world to me. Love you guys! - Z," he added.
--IANS (Posted on 29-09-2013)