Most guys treat working out much as they do calling Mom. It's a good thing to do regularly, but skipping it once in a while isn't going to cause irreparable harm. If, on a Thursday night after a 10-hour slog at the office, you have a choice between putting on your sneakers or ordering takeout from that new panini place, who's going to blame you for selecting the activity that doesn't involve sweaty socks?

But a growing number of guys, it seems, think that missing one day of exercise, or even cutting short a workout, will suddenly make them frail or—far worse—fat. Among their ranks is the guy who can't sleep nights because he's too amped-up about the training he's going to do at 5 a.m. If his trip to the gym isn't up to snuff, he'll log another in the evening, making excuses to blow off work or a date. And then there's the guy who feels compelled to ride the stationary bike until he has burned off every last calorie from lunch ("You're paying the price for that slice of pizza," he tells himself). And if these workout junkies can't exercise—if they spend a mere 24 hours away from the weight room—they plunge headlong into depression.

"There's a road to addiction," says Jay Cardiello, a New York City strength-and-conditioning specialist who has trained the likes of 50 Cent. "You think, 'I worked out for an hour today. I feel great. I wonder if I'll feel better after an hour and a half.' Then you stop hanging out with your friends. The gym becomes an obsession." Cardiello once had a client who would hit the weights for an hour before his hour-long personal-training session and then do an hour on a cardio machine—six days a week. "I wouldn't allow him in the gym anymore," he says. "I sent him home."

Health professionals call this behavior exercise dependency—the compulsion to work out, no matter what. It's also referred to as gym­orexia, exerciseorexia, and exercise bulimia. Estimates suggest that at least 20 percent of the gym-going public has a degree of exercise dependency. And when those affected can't have their drug of choice, like the average meth-head, they become despondent. "They get mood swings," says Jeremy Adams, a sports-and-exercise psychologist in London. "They get grumpy and irritated. They think, 'I'm going to lose all of my fitness. I'm going to get fat.'"

A couple of years ago, Michael Baccaro, 37, a recovering gymorexic in Fairfield, New Jersey, was lifting weights six or seven days a week and running about 35 miles a week. "That was awesome," he says wistfully. "I would love to get back to there. But it was consuming me." Every piece of cake or dollop of sour cream on a baked potato meant he had to tack on another five or so miles to his roadwork. He'd think, "Oh, man, I had fries today. I'd better work out." He rearranged his office schedule, taking a longer lunch or leaving early to run, and squirmed out of parenting duties so he could hit the health club (his gym-widow wife joked that he had a mistress there).

Exercise dependency is a relatively new disorder, brought on by two things: the spread of fitness-club culture and the increasing pressure for young men to be both lean and muscular—think Chace Crawford vs. The Situation. The modern gym-as-social-hot-spot, with people standing three-deep at the juice bar, has been around for only 30 years, but "if you think about exercise today, you think about the gym," Adams says. "This combined with more emphasis on looking a certain way makes working out a vital part of people's lives, and it's very easy to lose balance. You're going to find this problem anywhere that's big and cosmopolitan."