He’s one of NBC’s biggest success stories — keeping “The Tonight Show” at the top of the late-night ratings with bouncy show tunes and campy games.

Now Jimmy Fallon is also one of NBC’s biggest worries.

On Oct. 24, Fallon was stitched up at a hospital in Cambridge, Mass., after falling — and dropping a bottle of Jägermeister — while receiving the Harvard Lampoon’s Elmer Award for Excellence in Humor. “I threw the bottle and then I landed on broken glass,” a bandaged Fallon recapped on his show the following Monday. “There [were] pools of blood everywhere.”

It’s the party guy’s third injury in four months, and sources say NBC is worried that Fallon’s drinking has become excessive. And while they’re not doing anything about it — yet — “It’s gone from being a whisper to a chatter,” says one industry insider of Fallon’s over-the-top behavior.

In June, the 41-year-old comedian nearly severed his finger after tripping on a rug at his luxe Gramercy Park pad and catching his wedding ring on a countertop. Prior to that injury, he had been out dining and posing with wine-drinking fans at Upper East Side Italian restaurant Scalinatella.

“Tripped and caught my fall (good thing)!” he tweeted that day. “Ring caught on side of table almost ripped my finger off (bad thing).”

The hand injury required a six-hour surgery and a 10-day stay in the ICU.

Two months later, Fallon high-tailed it to the dentist for a chipped front tooth, which he says he got, ironically, while trying to open a jar of scar-tissue-repair gel to treat his damaged digit.

“#BestSummerEver,” he tweeted.

But not everyone is laughing.

The consecutive injuries come on the heels of a seemingly endless parade of Page Six mentions chronicling the after-hours antics of the host, who is married to Nancy Juvonen (co-owner with Drew Barrymore of the production company Flower Films) and has two daughters, both under 2 years old.

There was April 2014 at the East Village dive bar Niagara, where Fallon was allegedly involved in a brawl after a Thursday night taping of “The Tonight Show.” (The next evening, he managed to rally and partied at a Chelsea watering hole until the early morning, buying shots of vodka for the entire bar.)

This past January, patrons recorded a clearly inebriated Fallon belting out Katy Perry’s “Walking on Air” while literally crawling on the bar at West Village gay bar Marie’s Crisis.

An inside source says that before August’s annual Apollo in the Hamptons summer benefit, which is held at Ronald Perelman’s 57-acre East Hampton estate, Fallon was drinking heavily and “doing shots.”

“He’s a mess,” says a Manhattan bartender who has served the host over the years, adding, “I love him.”

She recalls a time around 2009 when a happily wasted Fallon, then the newly tapped host of “Late Night With Jimmy Fallon,” played beer pong with the staff at the now-closed Whiskey River in Union Square until 5:30 a.m., before continuing the party upstairs at the bar’s private apartment.

“Everything you’ve heard [about his behavior] is true,” she says.

No doubt, the star’s been an unabashed party boy since his “Saturday Night Live” days, frequenting anything-goes dives like Siberia, which was located in a subway station at West 50th Street.

“Everyone in New York has a Jimmy Fallon story,” Seth Herzog, Fallon’s friend and warm-up comic, told New York magazine in February 2014, right before Fallon ascended the “Tonight Show” throne.

“It’s always like, ‘In 2000, Jimmy threw this dude out of a bar, or poured his beer on someone, or got on a table and sang a song.’ ”

Fallon’s former “SNL” cast mate Horatio Sanz told the magazine that, back when they performed together, they “were superfunctioning alcoholics, definitely . . . They say that kind of goes hand in hand with ‘SNL,’ some kind of substance-abuse issues, because it’s so stressful, you easily find yourself blowing off steam a lot.”

No one argues with the fact that Fallon is a happy drunk.

“He likes to hang out and be the life of any party,” the bartender says, adding that Fallon now brings a bodyguard on his escapades and keeps a driver waiting to take him home.

“When he’s drunk, he doesn’t care who takes a photo of him.”

“Sources say NBC is worried that Fallon’s drinking has become excessive.”

A buddy adds: “Jimmy generally goes out with very, very close friends and people from the show. He’s not rolling up to 1Oak [a scene-y bottle-service club]. But he likes to drink. He likes to go out.”

Since his first taste of celebrity, Fallon’s been insistent on being a man of the people. He’s the famous guy from TV who regales the crowd at Bill’s Food & Drink in Midtown with show tunes; the goofy dude who swiped a Kevin Eubanks album cover from the bathroom of Los Dos Molinos in Union Square in 2009 and bragged about it on TV afterward (he was banned from the restaurant).

“Jimmy Fallon is the same guy that he was when I met him 15 years ago,” says Tracy Westmoreland, former owner of the now-shuttered Siberia.

But while Fallon hasn’t changed, his job sure has. And so have the pressures.

He tapes “The Tonight Show” at 30 Rock five days a week, often working up to 12 hours a day. The show’s pre-taped bits are done in the morning or after taping, which means sometimes he can be at Rockefeller Center until midnight, well past the typical 6 to 6:30 p.m. wrap time.

One source says Fallon deals with the newfound stress by hitting the bottle.

“There is so much money at stake, the pressure is enormous. [Besides doing the show daily], they’re looking for the next viral thing to hit. He has to do calls with affiliates and sponsors,” the source says.

“He has no outlet. He’s young. He’s still learning. You forget he hasn’t been doing this 10 years.”

But while none of Fallon’s predecessors or peers is perfect, they are considerably more discreet.

Take David Letterman, a famous recluse (and secret philanderer) who was whisked away to the ’burbs immediately after taping his show. Jay Leno is a straight shooter whose biggest vice appears to be his car collection. Even Fallon’s current colleagues — like fellow “SNL” alum Seth Meyers, who took over “Late Night” when Jimmy got bumped up to “The Tonight Show” — avoid any reports of fratlike antics.

One NBC insider, who asked to remain anonymous, says no one is sounding alarms because everyone views Jimmy as “a big kid.”

“He likes to have a good time. His show takes a lot of work: He has to learn songs and dances,” which would be hard to do if he were constantly wasted, they say.

“Jimmy Fallon is a highly valued member of the NBC family who is both a tireless worker and an extraordinarily gifted performer,” says an NBC spokesperson in an official statement.

“His dedication in putting on a great show each and every night is unparalleled, and that is reflected in the large amount of viewers who tune in — he frequently outrates the broadcast competition combined. We are proud of his accomplishments on and off camera and look forward to working with him for many years to come.”

In August, NBC extended Fallon’s contract an additional 3½ years to fall 2021.

If the October 2015 issue of Vanity Fair is any indication, Fallon is aware of his reputation.

In the magazine, he’s seen photographed alongside all the other men of late night — each of them holding a drink, save for Fallon.

“The Jimmy thing was a little interesting, because I don’t know that everyone got the memo that we were going to have drinks,” says Sam Jones, who photographed the TV hosts with bottles or glasses of liquor (and “The Late Late Show” host James Corden with a juice box, for giggles).

“We ended up shooting that a few different ways and . . . in the end he decided, ‘I don’t want to have a drink in the picture,’ ” says Jones.

The omission could speak volumes to those worried about Fallon.

But it doesn’t fix the inherent stresses of his high-ranking gig.

“Each one of those people . . . doesn’t really have anyone to share the pressures of the job with, or how difficult it is or how relentless it is,” says Jones.

“There’s a sort of loneliness to being a late night host.”