CLEAN-CUT “High School Musical” star ZAC EFRON collapsed after a “heroin” overdose – and The ENQUIRER has the shocking photo proof!

An exhaustive investigation has uncovered the stunning details about a drug-fueled party in the 25-year-old Disney actor’s New York City hotel suite ear­lier this year. After hours of drink­ing and abusing drugs, a source says Zac OD’d on the powerful painkiller oxyco­done – known on the streets as “hillbil­ly heroin” – and had to be revived by other guests.

The photo, taken in the wee hours of Jan. 3 at the Thompson Hotel, shows Zac out cold.

“I was scared for him,” an eye­witness told The ENQUIRER. “If he doesn’t clean himself up, he’ll be Hollywood’s next big drug tragedy.”

The eyewitness, who passed a polygraph test, said that Zac consumed massive quantities of oxycodone and the stimulant Ad­derall during the late-night bash. The source says Zac also smoked marijuana and was intoxicated from hours of drinking.

Zac’s drug use hit headlines a few weeks ago after it was revealed he’d completed a stint in rehab to deal with an addiction to cocaine. His abuse of the drug allegedly “spiraled out of control” during the filming of Seth Rogen’s movie “Neighbors.”

Published reports also said he had experimented with MDMA – a pure form of an ingre­dient found in Ecstasy tablets, which is known as “Molly” on the street.

Before heading up to his suite on the night of the overdose, Zac was said to be out partying with two female friends. Then he wanted to score oxyco­done pills, says the source.

“They had to drive around and go to two separate dealers because he wanted so many pills,” the source revealed.

The drug, a prescription pain­killer sold in tablets under the trade name OxyContin, got the nickname “hillbilly heroin” be­cause of its abuse in rural areas in the mid-’90s. A synthetic form of opioid analgesic, oxycodone is similar to morphine and is usually prescribed to cancer sufferers for their pain.

“Oxycodone is highly addictive,” Dr. Damon Raskin, an addiction expert and internist at the addic­tion treatment center Cliffside Malibu, told The ENQUIRER. “As the brain’s receptors become tol­erant of the drug, a person needs more and more to achieve the same high.”

On the street, the pills sell for $30 each, and the source says Zac requested that party guests buy 40 – at a cost of $1,200.

When guests arrived at the hotel, Zac apparently came to the door unshaven, wearing sweatpants and no shirt.

“If he didn’t have such a need for the oxy, I don’t think he would have let us come into the room,” the source said. “He was like a junkie. He wanted the drugs so bad, and this was the only way he could get them.”

The star’s hotel suite was messy and littered with empty alcohol bottles, says the source, adding that the partiers smoked marijua­na with Zac before they moved on to oxycodone.

“Most people crush it and snort one pill at a time, but Zac doubled that,” the source said. “He went through a lot of pills even though it was clear he was entering a dan­ger zone.”

According to the source, the actor then bought the ADHD med­ication Adderall from one of his guests, explaining that he needed it so that he could be alert the fol­lowing day on the set of his film “Are We Officially Dating?” But the source says that Zac swallowed those pills right away and admit­ted that he’d smoked pot and used “Molly” the night before.

Zac then disappeared into the bedroom with a 25-year-old bru­nette, and “she later told me they had sex,” said the source.

By 4 a.m., the actor was out cold, leaving his fellow partiers terrified.

“My worst fear was that he had died from an overdose,” the source said.

According to Dr. Raskin, who has not treated Zac, increased dos­es of oxycodone can lead to serious respiratory and cardiac issues – including a heart attack.

Fortunately, concerned partygo­ers were able to rouse Zac after repeatedly slapping his face, then dousing him with water.

He was lucky to fare better than “Glee” star Cory Monteith, who died of a heroin and alcohol over­dose in July, and Heath Ledger, the “Brokeback Mountain” hunk who accidently overdosed from pre­scription drugs in 2008.

“I’m glad to hear that Zac went to rehab,” the source added.” He clearly had a serious drug prob­lem.”