Can Pokemon Go improve your mental health? Maybe.

The game, which was released last week, has quickly become a viral social phenomenon, and already, dozens of players—called trainers– have taken to Twitter to tell the world how much better they feel after spending hours walking around chasing the virtual creatures that pop up on their smartphones.

Here’s a recent example:

HiRez David, it turns out, is a 26-year-old year events producer for a video game company in Alpharetta, GA. He describes himself as a “textbook introvert” who needs time to mentally psych himself up before hanging out with other people. A few years ago, he says he went through a rough patch and needed regular prescription meds to help him deal with some crippling anxiety.

So when he spent hours outside this weekend walking around and playing Pokemon Go with friends, it was definitely out of character.

“I spend more than 80% of my weekends at home, usually,” he says. “It’s really weird to think that I spent more time out of the house this weekend than I spent in the house.”

David says he spent four or five hours just meandering around a historic cemetery on Sunday, which is “800% more” exercise than he’d usually get during the day.

How does it work? The game is a free app that’s designed to be downloaded to a smartphone. It uses GPS and the phone’s camera to direct players, called “trainers”, to the Pokemon creatures that appear to pop up in the real world. Once alerted to a location, the game requires trainers to walk to capture the creatures.

Many fans say they’re already addicted, logging miles each day on a quest to “catch them all.” Others are complaining about sore muscles from all the exercise they’re getting.

Experts say the mental health benefits many players are reporting aren’t surprising. Exercise has long been shown to be a mood booster.

“These stories make perfect sense,” says Pamela Rutledge, PhD, director of the nonprofit Media Psychology Research Center in Newport Beach, CA. She says there’s a lot to like about Pokemon Go.

“Aside from social contact and activity—both known to increase positive mood, decrease anxiety and depression, and improve immune systems, people who go outside are exposed to sunlight and receive the benefits of Vitamin D,” she says.

The game also uses local monuments and buildings as PokeStops—points integral to the game—and that encourages people to learn about their local environment.

Using your smartphone to chase Pokemon can have some downsides, though.

Rutledge says there’s the danger of selective attention, or not paying attention to where you are when you’re playing. And as with any guilty pleasure, she says too much Pokemon Go can be unhealthy if it leads to new ways to procrastinate when you should be doing something else.

Other safety concerns have cropped up, too. Tech savvy thieves in Missouri and Pennsylvania have reportedly exploited features of the game to lure and rob unsuspecting players. And while the game is designed to be played on foot, some people are doing it behind the wheel, which makes for a dangerous distraction on the road.

Follow these tips from the St. George Utah police department to stay safe while Pokemon Go-ing:

• Don’t wander onto private property. “Just because there’s a Pokemon there doesn’t mean YOU can be there. Never enter another person’s property without permission.”
• Beware of the “lure modules,” which thieves have exploited to rob people. “If at all possible: Don’t ‘Pokemon Go’ alone.”
• Don’t play while driving.
• Be aware of your surroundings while walking and playing.