Are these supplements good fuel for a gym session—or can they cause real harm?

Did you catch the latest controversy on The Biggest Loser? Last week, trainer Jillian Michaels gave her team caffeine supplements before a workout without a doctor’s permission. The team was penalized for it, but Michaels stands by the supps, saying that they’re effective. “A caffeine supplement is significantly healthier than unlimited amounts of coffee,” she said on the show.

So what’s the deal? Michaels isn’t wrong—caffeine in any form can boost energy and improve performance, says Nancy Clark, MS, RD, a Boston-area nutritionist and author of Nancy Clark’s Sports Nutrition Guidebook. Problem is, one caffeine pill might contain more of the drug than a person needs.

“A recommended target is about 1.5 mg of caffeine per pound of body weight,” says Clark, and that's the high end of sports nutrition guidelines. So if you weigh 150 pounds, that equates to 225 mg of caffeine—about 12 to 18 ounces of coffee. A pill that contains 200 mg of caffeine is close to that target but if you've already had coffee that day, or if you take a caffeine supplement before an afternoon workout, you might be awake all night, Clark warns.

Ultimately, too much caffeine can also lead to a caffeine overdose—a real condition characterized by symptoms such as irritability, dizziness, rapid heartbeat, and mental fogginess. Any of these can make you feel lousy, not to mention screw up your plan to kill it on the treadmill.

Considering the risk of these scary symptoms, and the fact that dosing up on caffeine might leave you too wired to sleep at night, Clark doesn't recommend relying on caffeine in any form and instead suggests jumpstarting your energy by eating healthy foods.