No effects or prosthetics were created for Stu's missing tooth. Ed Helms never had an adult incisor grow, and his fake incisor was taken out for the parts of filming where Stu's tooth is missing.

Ed Helms' and Zach Galifianakis' songs ("Stu's Song" and "Three Best Friends", respectively) were improvised.

When the cop car pulls up in front of Caesar's Palace, two real employees are having an argument in the background about the filming of the movie.

A real taser was originally going to be used in the taser scene, but lawyers from Warner Brothers demanded that the crew use a prop.

In real life, Mike Tyson owns seven tigers.

Mike Tyson revealed that he appeared in the film to fund his drug habit, and that he was high on cocaine when he filmed his scenes. Tyson later said that working on the film convinced him to change his lifestyle.

Ironically, Bradley Cooper is a teetotaler in real life. He quit drinking at the age of twenty-nine.

Lindsay Lohan was offered the part of Jade, the stripper, but turned it down. She did not like the script and thought the movie would bomb. Heather Graham was given the role instead. Lohan later admitted to regretting the decision.

During filming in Las Vegas, one of the Mercedes (a very beat up and distinctive one) was stolen from the lot where the vehicles were being kept. The next day, the production was filming driving sequences, and traffic was being held up by the local police. A production person noticed that a very distinctive Mercedes was part of the cars being held, the police were told, the driver was arrested, and the car was recovered.

Highest grossing R-rated comedy in the U.S. to its release date.

By depicting and celebrating Las Vegas as the "ultimate guys' getaway," the film had a major impact on Caesar's Palace and Las Vegas. It was reported in 2013 that as of that year, guests continue to quote to Caesars staff two lines from the film's check-in scene: "Did Caesar live here?" and "Do you know if the hotel is pager friendly?" As a result of the film, Hangover-themed slot machines became popular at casinos throughout the Las Vegas valley, the Caesar's Palace gift shop sold tens of thousands of Hangover-related souvenirs, and the Las Vegas Convention and Visitors Authority received numerous inquiries from people interested in re-creating some of the film's most wild scenes, such as those involving a tiger.

Zach Galifianakis said he tried to forbid his own mother from seeing the film, and that he yells at parents of kids who tell him they like it.

Paul Rudd turned down the role of Phil Wenneck.

Jack Black turned down the role of Alan Garner and Jonah Hill, Jake Gyllenhaal, and Thomas Haden Church were considered for the role before it was substantially re-written during development.

The plot was inspired by a real event that happened to Tripp Vinson, a producer and friend of executive producer Chris Bender. Vinson had gone missing from his own Las Vegas bachelor party, blacking out and waking up "in a strip club being threatened with a very, very large bill I was supposed to pay."

Ed Helms, Zach Galifianakis, and Bradley Cooper were all casual acquaintances before the movie was filmed, which Helms said he believed helped in establishing a rapport and chemistry amongst their characters. Helms credited director Todd Phillips for "bringing together three guys who are really different, but really appreciate each others' humor and sensibilities." Helms also said the fact that the story of the three characters growing closer and bonding informed the friendship between the three actors: "As you spend fourteen hours a day together for three months, you see a lot of sides of somebody. We went through the wringer together, and that shared experience really made us genuine buddies."

Plans for a sequel (which became The Hangover Part II (2011)) had already been made before this film was even in theaters.

Three sets of twins and a dummy were used to portray the baby.

Zach Galifianakis improvised the line "I didn't know they gave out rings at the Holocaust". The initial line had Alan making a sexually inappropriate comment about Stu's mom, but Zach thought up the new line for two reasons: he wanted to show that Alan was someone who knew almost nothing about the world, and that a successful joke that mentioned the Holocaust would establish that the film was going to be hilarious overall. Everyone on-set loved the joke (Bradley Cooper appears to be disgusted by it, but he was actually following the script's directions for Phil to look at Alan in contempt, after the original line that was written out) and Todd Phillips said it would be in the film.

Director Todd Phillips waived his usual salary and received part ownership of the film instead. In an interview with Howard Stern, Phillips said that due to the film's box office success, "this is my Star Wars: Episode IV - A New Hope (1977)."

Mike Tyson originally refused to appear in the film, but he changed his mind when he found out that Todd Phillips directed Old School (2003), which Tyson liked.

Ed Helms said that the most difficult day of shooting was the scene when Mr. Chow rams his car and attacks the main characters, which Helms said required many takes and was very painful, such as when a few of the punches and kicks accidentally landed and when his knees and shins were hurt while being pulled out of a window.

On the DVD commentary, Zach Galifianakis gave an explanation as to why Alan is not allowed to be within two hundred feet of an elementary school. He innocently tried to play with some children, which of course looked suspicious to some parents. It was all just a misunderstanding.

Despite Bradley Cooper's character heavily drinking in this film, he hasn't touched a drink since he was 29 years old.

Ken Jeong's wife Tran was battling breast cancer and undergoing chemotherapy treatment, which was difficult for the family as Ken was shooting the film in Las Vegas. Bradley Cooper drove Ken to Los Angeles so he could be there for his wife. Cooper invited the couple to spend the holidays with him and his family. Ken Jeong said " "It got me through the most difficult period of my life." Tran Jeong was declared cancer free after the film wrapped.

The screenplay for this film was featured in the 2007 Blacklist; a list of the "most liked" unmade scripts of the year.

When the cop calls Alan "fat Jesus", this is a reference to Zach Galifianakis' stand up, "I look like a fat Jesus."

Amusingly, Alan is actually correct - card counting is NOT illegal, and is indeed only frowned upon. Casinos are not allowed to ban suspected counters, as per the Supreme Court ruling, and must rely on internal countermeasures to stop them.

One of the songs from the movie is "Can't Tell Me Nothing" by Kanye West. One of the stars, Zach Galifianakis was paid by West to make an alternative music video for the song.

On a budget of $35 million, principal photography took place in Nevada for fifteen days.

Josh Lucas, Vince Vaughn, Josh Hartnett, and Ethan Embry were considered for the part of Phil Wenneck, while Breckin Meyer and Seth Rogen were considered for the part of Stu Price.

Regarding the explicit shots in the final photo slide show in which his character is seen receiving fellatio in an elevator, Zach Galifianakis confirmed that a prosthesis was used for the scene, and that he had been more embarrassed than anyone else during the creation of the shot. "You would think that I wouldn't be the one who was embarrassed; I was extremely embarrassed. I really didn't even want it in there. I offered Todd's assistant a lot of money to convince him to take it out of the movie. I did. But it made it in there."

The scenes involving animals were filmed mostly with trained animals. Trainers and safety equipment were digitally removed from the final version. Some prop animals were used, such as when the tiger was hidden under a sheet and being moved on a baggage cart. Such efforts were given an "Outstanding" rating by the American Humane Association for the monitoring and treatment of the animals.

During the scene where Phil, Stu, and Alan meet Chow in the desert, you can see their car pulling up in the reflection of Chow's sunglasses. This is a tribute to the scene in Casino (1995) where Nicky Santoro (Joe Pesci) meets Sam Rothstein (Robert De Niro) in the desert and you see the reflection of Santoro's car in Rothstein's sunglasses.

According to Celena Haas (Caesar's Palace Director of Public Relations), the suite the characters stayed at was modelled after two of the resort's luxurious suites in its Forum Tower: a well-appointed penthouse, and the so-called "Rain Man (1988) suite," located on the 68th and 69th floors of the Forum Tower.

Jeremy Piven turned down the part of Stu Price.

The song that plays during the scene in which Neeco, the tuxedo deliveryman, tosses the guys' rented tuxedos into their speeding car is "Ride the Sky II" by Revolution Mother. Neeco was played by Mike Vallely, Revolution Mother's lead singer, and a pro skateboarder.

Casino scene, where Alan and Phil go down the escalator, is a tribute replicating famous scene from Rain Man (1988) where Tom Cruise and Dustin Hoffman go to the casino.

In the scene when they are walking to Jade's apartment, Alan refers to a movie with Ted Danson, Magnum, P.I. (1980), and "that Jewish guy". This refers to Three Men and a Baby (1987). "That Jewish guy" was Steve Guttenberg.

Will Ferrell was considered for Stu Price.

Alan and Tracy's father Sid's car is a 1965 Mercedes-Benz 220SE (W111).

Due to the physicality of his role, Ed Helms lost a reported eight pounds.

Included among the "1001 Movies You Must See Before You Die", edited by Steven Schneider.

Ed Helms (Stu), Rachael Harris (Melissa), Rob Riggle (Officer Franklin), and Matt Walsh (Dr. Valsh) appeared as correspondents on The Daily Show (1996).

Each film features a different song by the heavy metal band Danzig, this movie: Thirteen, Hangover II: Black Hell, Hangover III: Mother, the band's only top 40 single.

Bradley Cooper and Justin Bartha appeared in Failure to Launch (2006) and New York, I Love You (2008).

Malin Akerman had been considered for the role of Jade.

The wedding singer played by Dan Finnerty was also the wedding singer in Old School (2003), also directed by Todd Phillips.

One of five films featuring both Bradley Cooper and Ken Jeong. Both have appeared in "The Hangover" trilogy, "All About Steve" and "Avengers: Endgame", although in that last film, Jeong makes a cameo as a security guard and doesn't share a scene with Cooper's character.

Cleo King, who plays a police officer who busts into Jade's apartment, also played a cop in Pineapple Express (2008).

In the first draft of the script, Bradley Cooper's character name was Vick.

When the four friends hit the road, Alan (Zach Galifianakis) screams "road trip!" Todd Phillips directed Road Trip (2000).

Before this film, Bradley Cooper was a guest on Zach Galifianakis' short-lived talk show, "Late World With Zach" on VH1.

As a tribute to the film, Rascal Flatts took to Vegas to film their video for "Why Wait", which featured cameos from Scott "Carrot Top" Thompson and Wayne Newton.

The stolen cop car is a 2000 Ford Crown Victoria [P71].

An original draft of the script had Alan in a disastrous relationship, Stu being married with two daughters, and Vick (Phil's original name) being a wild party boy.

The song 'Take it off' by The Donnas is playing when the guys first drive to Vegas. The same song was used in season 3, episode 10 of the tv-show 'Alias'. Bradley Cooper, who had left the show after season 2 made a comeback to this particular episode and the song is playing in a scene where his character, Will Tippin is breaking into a hotel safety box with Sydney Bristow (Jennifer Garner.)

Todd Phillips and Bradley Cooper's first film together.

The taser was an X-26, which fires two small barbed darts intended to puncture the skin and remain attached to the target. The darts are connected to the main unit by thin insulated copper wire and deliver a modulated electric current designed to disrupt voluntary control of muscles, causing "neuromuscular incapacitation." The effects of a TASER may only be localized pain or strong involuntary long muscle contractions, based on the mode of use and connectivity of the darts. The TASER is marketed as less-lethal since the possibility of serious injury or death exists whenever the weapon is deployed. The electrical output of the TASER is 50,000 Volts. The voltage may seem high, but the amperage on both systems is well below safe limits. ADVANCED TASER M26 output is 3.6mA average current (0.0036 Amps) The X26 output is 2.1mA (0.0021 Amps).

When the lady in the elevator asks what the baby's name is, Phil (Bradley Cooper) says it's Ben. Bradley's character's name in "He's Just Not That Into You" is Ben. Both movies were released in 2009.

Zach Galifianakis: [Chuck E. Cheese] Alan makes a joke about him not being allowed near schools or a Chuck E. Cheese, a direct reference to one of his staple jokes in his stand up: "When you look like I do, it's hard to get a table for one at Chuck E. Cheese."

Lucinda Jenney: At the casino, playing the same character she played in Rain Man (1988).

Todd Phillips: A guy making out with a woman in an elevator at the hotel.

Todd Phillips: [band] The foul-mouthed band featured near the end of the movie is the "Dan Band", the same band from Frank's wedding in the movie Old School (2003) and Vince Vaughn's daughters' Bat Mitzvah in Starsky & Hutch (2004).

Ken Jeong improvised the nudity in the scene where he jumps out naked from the trunk. Originally, his character was meant to have clothes on, but Ken suggested that his character be naked instead. Later, director Todd Phillips gave Ken a nudity waiver slip to sign in case Ken changed his mind on the nude scene.

According to Bradley Cooper, while giving a speech at a Toronto screening: "the people in Las Vegas never even noticed a movie was being filmed." He said that he had tiger scratches on his neck, all bloodied, and Justin Bartha was beet red, and nobody noticed, or even stopped to double-take.

In the DVD commentary, Todd Phillips stated that a possible explanation for the chicken in the room is that the guys intended to feed it to the tiger.

In the MPAA green-band trailer, Ken Jeong is wearing digitally-added underpants when he jumps out of the trunk, and Bradley Cooper says "paging Dr. Douchebag" to Ed Helms instead of "paging Dr. Faggot".

The escalator scene in the casino is a tribute to Rain Man (1988). Alan and Phil have to win eighty thousand dollars in order to pay back Mr Chow. This was also the amount needed to win by Raymond and Charlie in Rain Man (1988).

In the taser scene when the Officer is loading cartridge into the taser he puts his hand over the front. Police are trained to not put anything in front of the blast doors in case the probes launch when it connects.