On the label of 5-Hour Energy—a fatigue-fighting drink that claims to be a healthy alternative to sugar bombs like Red Bull, Monster, and venti Frappuccinos—a silhouetted runner bounds up a mountain at sunset. I received the sample, which comes in a two-ounce plastic bottle, after spending a deskbound summer under a brutal book-project deadline. I looked wistfully at the image of the runner. When was the last time I'd felt like that? I read the label. FOR MODERATE ENERGY: DRINK A HALF BOTTLE OR LESS ... FOR MAXIMUM ENERGY: DRINK ENTIRE BOTTLE AT ONE TIME.

I tend to be skeptical of such products, but suddenly I found myself tearing off the cap and gulping the contents, the ultra-tart liquid making a little glick-glick-glick sound as it emptied into my mouth. It was around 2 p.m., and that afternoon I finished writing a book chapter, did three loads of laundry, cleaned and organized my office, and took my dog out for an hourlong trail run. Shazam—I was that guy on the bottle!

The boost behind the juice comes from large doses of B vitamins and a proprietary energy blend that includes, among other things, caffeine, taurine, and phenylalanine. I tried 5-Hour again the next day, with similar results; I even scored the winning goal in my weekly soccer game.

I was so amazed by the jolt that I called up sports dietitian Dave Ellis, who works with pros from the NFL and NBA. He hadn't tried 5-Hour, but he wasn't impressed. "You build a tolerance to these stimulants, and when you can't feel one, you go to two; when you can't feel two, you go to three," he said. "That creates a cycle of codependence."

Living Essentials, the maker of 5-Hour, claims it's not just the caffeine but the large hit of B vitamins that provides the punch. Each shot contains 40 milligrams of vitamin B6 and 500 micrograms of B12—respectively, 2,000 and 8,333 percent of the recommended daily allowances. It's long been known that B vitamins aid the metabolism and sharpen mental acuity; my grandfather, a physician, used to give my grandmother B12 shots to crank up her energy. "The problem is, we don't know what happens over time when you isolate nutrients," Ellis said. "The combination of nutrients in real food offers benefits that we're just beginning to discover."

I tried 5-Hour for a couple of weeks, in various circumstances, and, sure enough, I soon started craving a double dose. Hmmm. I was on a slippery slope when what I really wanted was to be back on the trail.