Do they really drink or eat when shooting movies?

They can use something called Spit Bucket.

Actor Nick Offerman (Parks and Recreation) explains:

When you consume something in a scene, you learn to be very circumspect because you may end up having to consume that amount 30 times.

When you're young, you think, "Man, it would be really cool if I smoke in this scene." Then, on take 18, you're green and vomiting because you've been smoking for 90 minutes.

So you learn with food: If you have to take a bite of something, don't swallow it, and get a spit bucket.

Even if you're eating one bite of cake, you may end up eating four pieces of cake by the time you add them all up. It makes sense, especially if you're attractive. I have the good fortune of not being included in that category.

Julia Roberts revealed in an interview about her movie Eat Pray Love that she doesn't like a spit bucket:

Did you have a spit bucket by your side for all of the food scenes?

• Well, first of all, that grosses me out. But the truth of the matter is, there probably would have come a point when I would have used it.

  If you look at any of the scenes of eating, by the end of the scene, I’m done eating. Like in the scene with the pizza, by the time the scene is over, I’ve eaten the entire piece.

  When we were in Naples, we started shooting at 8 in the morning, and I think by 8:45 I’d eaten 8 or 10 pieces of pizza. Pizza was what I ate all day that day.

What Are Actors Actually Snorting, Smoking And Drinking In Films?

Most of the time, it's not the actual substances...

Regarding Alcohol:

• Beer: Carbonated iced tea or "near beer" (less than 0.5% alcohol)

• Dark colored liquors: Weak tea

• Clear liquids: Water

Click the link to learn what substitutes they use for cocaine, marijuana and cigarettes (and for which movie Nicolas Cage snorted real cocaine)

Sometimes yes sometimes no but what they eating in reality you never know.

Fake Food

Sometimes Food has often been improved for the purpose of shooting. Like Lettuce will always be crisp, hamburger buns perfect and full, cake light and fluffy. Ice cream is never melting, unless that was the point. As good as the food may be made to look, however, actually eating that specific example is probably hazardous to your health.

Arrange the items on the plate so that (at the angle chosen for the shot), where the view is blocked by other food, there's nothing behind it - but people won't realize that! Cut a wedge out of the hamburger patty, so you can spread it out a bit, making it look wider. Use camera angles and zooms that make it look larger than it actually is - essentially.

For a good example of the last of those tricks, watch a Dairy Queen ad for their Peanut Buster Parfait and compare how large it appears with what you actually get in the restaurant. You will notice that you rarely see an actor's hands in the picture to give you a sense of scale.

Ice cream sundaes are often constructed of scoops of lard or mashed potato covered in motor oil, or other toxic-yet-pretty trickery. Likewise, "steam" rising from "hot" food is often smoke from a hidden cigarette, and ice cubes will really be deftly sculpted chunks of acrylic.

(Source: tvtropes)

Some Examples from Films and Television are :

Film

• Big Night was set in an Italian restaurant, and all the food was undercooked to look better on screen. The actors would spit it out between takes.
• In one notable scene in Animal House, John Belushi's character downs an entire bottle of whiskey; reportedly, the bottle was actually filled with tea.
• On the first Harry Potter movie, Chris Columbus insisted on using all real food for the Great Hall. This did not go well, as they had to replace food whenever it spoiled, essentially forcing them to churn out the Hogwarts feast over and over again. In subsequent films, much of the Great Hall food was cast out of resin.

Television

• Exception: in Coupling, actor Ben Miles decided that his character Patrick was a Guinness drinker. This may have had something to do with the fact that Guinness is very difficult to fake convincingly, so while his fellow actors were drinking coloured water with foam on top, he was drinking genuine Guinness...

• The giant cup of the "Caf-Pow" soda that NCIS lab rat Abby frequently drinks is actually sugar-free cranberry juice; it was orginally Hawaiian Punch up until around the fourth season, when Pauley Perrette gave up eating or drinking anything that contained refined sugar.

• In an episode of Stargate SG-1 called Window of Opportunity, O'Neill and Teal'c repeat the same 10-hour period over and over again. The period started with O'Neill eating Fruit Loops, and the start of each period began with the spoonful of Fruit Loops halfway to his mouth. In the DVD commentary, it was revealed that the Fruit Loops were glued to the spoon, to guarantee they'd be in the same place in every shot.

• The Monkees episode "Success Story", while pretending to be rich and successful, Davy is served rubber food while his grandfather gets the real stuff. Also, the fruit in the center of the table is plastic.

• When he hosted Saturday Night Live, Ron Howard talked about drinking fake beer while filming Happy Days in his openning monologue. He then pulled out a can, declaring it to be the real thing, then chugging it.

• In episode 10 of Season 17 of The Amazing Race, teams were sent to a restaurant in Japan's Kappabashi-dori district, and had to pick out the fake food from a buffet table covered with real and fake display-food items. The task took some teams several hours, and a couple of racers picked wrong (real food) so often that they got sick from the amount they had to eat.

• Actors from the classic British television series Upstairs Downstairs have stated that, while those performers playing servants (the "Downstairs" characters) often had real, good food to eat in meal scenes, those portraying members of the family upstairs had things like "grouse that had gone off" and stuff painted or coated in glycerin and other product to make it look "perfect."

Full details are available on tvtropes.

To allow for maximum flexibility in editing, it is a good idea to not to consume any drinks and food. Since scenes are shot several times from different angles, the editor wants to be able to choose which takes to put together, without worrying about having to ensure that the level of a glass of wine was not changing often radically during an edit point.

One exception would be if the scene was a very long scene at a restaurant then one would have transition points where the amount of food/beverages were reduced to show the passage of time. Even in this case there would be plates/glasses with various amount of food in them. This would normally need to be carefully monitored as the entire scene would be shot form one angle, and then reshot from an alternate angle. It takes a lot of time to move the camera and set up the lighting, but not much time to replace the plates/glasses with different amounts in them. This would definitely be the case for large budget movies. In sitcoms and low budge movies many short cuts could be taken to keep the costs down.

So, unless the complete purpose of the scene is to consume food, or in low budget movies, I'd say they don't actually eat or drink. Doesn't mean that there won't be exceptions to the rule though.

So far in my experience as an extra, beer is real but 0% alcohol and varying taste. Red wine is usually Ribena or other kind a blackberry juice, white wine apple juice, clear drinks (Vodka) water, whiskies are cold tea, and so on. Food I found is generally real. Cigarettes are (clean) herbal, have been since 1998. Blood is sugar syrup in some cases but generally food based.