ABC News' Reena Ninan reports:

Walk down any sidewalk or ride on any subway train and there's a good chance you'll see people glued to their smartphones strategically trying to swipe candy pieces from one space to another.

The game, available through a free app, is called Candy Crush Saga, and it's an addiction people are apparently willing to spend a lot of money on.

"When my husband told me our iTunes total, he was like, 'You've got to stop,'" Mary Hilton told ABC News.

Hilton, of Steilacoom, Wash., is one of the approximately 45 million people who play Candy Crush Saga on Facebook each month.

The game, in which players must move the candy pieces to line up three of the same type in a row to crush them or make them disappear in order to start all over, is the most downloaded mobile game on both Android and Apple devices, according to industry data.

Candy Crush Saga is also the top-grossing mobile app because, while it is free to download and play, you can purchase additional lives to stay in the game. Think Gaming estimates that Candy Crush brings in around $633,000 per day for King.com, its Britain-based maker, or $231 million per year in the U.S. alone.

Hilton says she has spent the past year, and a lot of money, getting to level 245, out of 400, in the game.

"I got hooked right away," she said.

Hilton is right in the game's demographic, according to King, which says women ages 25 to 55 are its most loyal customers, reports the U.K.'s Daily Mail.

"Every time you reach a certain level you get a sense of satisfaction," said Dr. Jeffrey Gardere, assistant clinical professor at Touro College of Osteopathic Medicine in New York. "We're listening to music, which is very soothing, as we're playing it [and] we hear a male voice that's giving us all this positive reinforcement."

Denise D'Andrea is another woman in the Candy Crush Saga target demographic who finds herself unable to stop playing the game, even while on vacation.

"I was on vacation in Cancun, and I'd find myself leaving the pool to go back to the room, and I would sit there and I'd be playing Candy Crush trying to get to the next level," she said. "I'm a little addicted to it."

Gardere compares the rush women like D'Andrea and Hilton feel when they play the game to what others feel when they gamble.

"[They're] waiting for the payoff," he said. "That's the same thing that happens with gambling."