“Saturday Night Live,” now in its 50th season, kicked off broadcast life with a blast – of illicit behavior.

The 17th floor offices at 30 Rock in Midtown Manhattan were rife with drug use and casual sex among the cast and crew. Pot smoke permeated the air, dressing rooms were used for quickies and at least one cast member, Garrett Morris, came to maintain a not-so-funny freebase-cocaine addiction. Fittingly, sketches often referenced recreational drugs.

Some members of the “SNL” band, led by Howard Shore — who told The Post that he does not recall anything about outrageous behavior on the show — knew which substances were best tolerated by the program’s brass.

“They used to smoke weed on the bandstand during the show,” a former SNL staffer told The Post, adding that some of the musicians indulged in alcohol as well. “Then they got a memo from [show creator] Lorne Michaels one day. It said, ‘From now on there will be no drinking on the bandstand. It sets a bad precedent for the union stagehands. Stick with the drugs.’”

When the first episode dropped in October 1975, it was the start of a show that nobody could have imagined would become the longest running hit on television. “We thought it could be one and over,” a guest who appeared during the first season of “SNL,” told The Post. “There was a lot of stress and tension over how well it would be received.”

The cast of that first season on opening night was made up of: Chevy Chase, John Belushi, Dan Aykroyd, Gilda Radner, Garrett Morris, Jane Curtin, Laraine Newman and Michael O’Donoghue.

The lead up to the first show was so loaded with drama Jason Reitman took the 90 minutes before curtain time and made it into the new movie “Saturday Night,” appearing in theaters on October 11.

“The dress rehearsal ran way over-length” Jeff Weingrad, coauthor, with Doug Hill, of “Saturday Night: A Backstage History of Saturday Night Live,” told The Post. “Billy Crystal was going to get his big break doing a six-minute monologue. But Lorne Michaels said it would need to be cut to two minutes. Billy’s manager got very aggressive, in Lorne’s face, and said that if it gets cut, he’s not doing the show. Lorne said that was fine. Billy rode home on the Long Island Railroad while the show was going on.”

Back in ’75 a Rolling Stone reporter got things exactly right when he described “Saturday Night Live” as a “head show, one to get high before, during and on, as high as its actors clearly are.”

Cast members clearly got the memo and then some. Tom Malone told The Post about the band’s hangout room, which was known as the Departure Lounge for good reason. “It’s where we smoked our reefer,” he told The Post. “A lot of s–t used to happen in that room. Let’s just leave it at that.”

While some simply smoked pot and indulged in the free love that hung over from the 1960s – “The only entrée to that boys club was basically by f—ing somebody in the club,” Annie Beatts, a writer on the show who was going out with head-writer Michael O’Donoghue, said in “Live From New York” by Tom Shales and James Andrew Miller.

Others took drugs way beyond the relative innocence of marijuana. According to the book, white powders were common and there were cast members who dropped acid in the lobby of the RCA building where they could groove on the art deco detailing.

“Early on, Chevy Chase was one of the first to dabble in cocaine,” Weingrad said. “He [also] had an excess of ego and it rubbed many people on the show the wrong way. Hyping up his personality with cocaine only made it worse.”

Jibing with the staffer, who commented that as the performers’ paychecks got bigger, the drugs got better, Weingrad added, “Several of those on the show said that Chase was the first to start using cocaine because he was the first to be able to truly afford it.”

So much so that his blow was up for grabs – whether he liked it or not. During a recent appearance on Bill Maher’s podcast, Chase (who bailed from “SNL” after that first season made him into a household name) recalled his vial of cocaine going missing and suspecting that John Belushi stole it from him.

Belushi denied it. Chase recounted that, a month later, he was invited to dinner at Belushi’s apartment on Morton Street and saw “my little vial, empty and washed, just sitting on a shelf by the books.”

Representatives for Chevy Chase did not respond to a request for comment.

Chase was not the only one who lost coke to Belushi. “He asked [one of the cast members] if he had any coke,” remembered the staffer. “[The cast member] took out a vial that was practically empty. He said, ‘I just have a little bit of dust.’ Belushi said, ‘That won’t be enough to get me going. I’ll ask someone else.’

“Then, after John walked away, [the cast member] took out a full vial. I said to him, ‘I thought you were out.’ He said, ‘That was my Belushi blow.’ The point was that if you handed John a full vial of cocaine, he would dump it onto his hand and just do it all. I saw him lay out an eight-ball, which is 3.5 grams of coke, on a flat surface. He made big, long line, rolled up a $100 bill and did the whole thing in three minutes. It did not noticeably affect him.”

A band member recalled meeting Belushi in the Morton Street pad for a business chat. “After 40 minutes,” he said, “the business was over. John took six Quaaludes and chewed them down. Then he opened a bottle of Courvoisier and started chugging it. He handed me the bottle, half empty. What was I supposed to do?”

The guest from early on pointed out that while the drug use was rampant, it was also therapeutic: “The hours they worked and the pressure they were under was intense. There was competition to get on air, every week a new show needed to be put together and, because of the guest hosts, they had to get used to new personalities. It was a lot. The drugs were one way for people to be able to handle the environment.”

Even so, SNL’s road to excesses led to some bad places.

By 1982, Belushi fatally ODed on a lethal mix of heroin and cocaine. In “Live From New York,” Laraine Newman talked about hanging out in the home of Gilda Radner: “I would be snorting heroin,” Newman — the mother of “Hacks” star Hannah Einbinder — said in the book. “And [Radner] would be eating a gallon of ice cream. I remember her staggering to the bathroom to make herself throw up.”

In the meantime, Garrett Morris’s freebasing got so bad, according to Weingrad, that “he was always late for [rehearsal] on Saturday night … He claimed that an invisible hypnotist robot was controlling him.” In “Live From New York” Andrew Smith, a writer on the show, recalled his office is “where [Morris] used to freebase” and the maid was afraid to clean there.

Through it all, though, 50 years after the debut of “SNL,” it lives on as the show that woke up a sleepy medium. “With ‘Saturday Night Live,’ the generation that had grown up on television, finally got a show that they felt was their own,” said Weingrad. “And they embraced it.”