LOS ANGELES — Thousands of moviegoers were no doubt clutching their seats while watching “Cloverfield,” last weekend’s No. 1 film at the box office. At least a few of them were clutching their stomachs as well.

Since the movie opened Jan. 18, some patrons say they felt nauseated and dizzy while watching the horror flick, much of which was filmed with a hand-held camera, producing a shaky view.

Erika Hasegawa, 32, was watching “Cloverfield” at a theater in the Los Angeles-area community of Alhambra on Tuesday night but had to leave in the middle of the film.

“I’m really nauseous right now — just hold on for a second,” she said, before walking down the hall and retching in a trash can.

“I wish I could get my money back,” she said.

It’s unclear how many people have felt ill while watching the movie, which follows a group of young hipsters filming themselves with a camcorder while they flee a reptilian monster destroying New York City. The movie set box-office records on its holiday weekend opening, earning about $46 million.

Reports of illness while watching “Cloverfield” started popping up on Internet bulletin boards over the weekend, with some writing that they had to leave a few minutes into the movie, while others said they tried to stare at a wall as the movie continued. One Internet poster reported vomiting several times.

A call to Paramount Pictures, which released the movie, was not returned.

Some movie chains stressed that the vast majority of viewers felt fine watching the movie.

Nonetheless, the Kansas City-based AMC Theatres placed caution signs in hundreds of its theaters nationwide warning about possible motion sickness issues.

Mann Theatres Chief Executive Peter Dobson said there were three reports of illness at Mann Theatres in Los Angeles over the weekend and one report in nearby Glendale. “I must confess I was a little surprised, but sometimes from time to time this happens,” Dobson said, who added: “It’s not normal to get four in a weekend.”

Some experts were not surprised, given the film’s use of hand-held cameras that were jerked around to boost suspense.

It’s a technique that has been used in other movies — notably the “Blair Witch Project,” a film released in 1999 that also prompted viewer complaints about nausea and vomiting. Both movies were filmed from the perspective of young adults holding camcorders while trying to escape from a mysterious, terrifying force.

The problems may be more pronounced at theaters with oversize movie screens. The larger the screen, the harder it is for the viewer to keep a visual frame of reference, said John Risey, an audiologist at the Ochsner Clinic in New Orleans.

“To your brain, it does appear that the entire world is moving,” Risey said. That problem is not an issue when viewing the same film on a small TV screen, he said.

What the eyes process visually has enormous impact on how the body reacts, according to Dr. Dennis Maceri, an associate professor at the University of Southern California School of Medicine. The body reacts to what we see, even if it’s not real.

In the 1968 Steve McQueen film “Bullitt,” for example, viewers follow a camera filming a car chase set along the rolling hills of San Francisco.

“He goes down the big hill, and you can feel your stomach move … even though your body is stationary,” Maceri said. “The eyes can fool you.”

It’s similar to the feeling some people get while they try to read in a moving car, he said. To the eyes, the words on the page appear to jump around, and “you can’t stabilize your gaze.”

Peter Bohlen, an 18-year-old high school student from Glendale, said he felt nauseated after he watched the movie Jan. 19 with four of his friends, and still wasn’t feeling well a day later.

“They’re trying to go for the home video feel, so it’s constantly shaking around,” Bohlen said. “My brain tried to make it work, but it couldn’t. I got a pretty heinous headache afterward.”