Remember the days when a quick pick-me-up meant you reached for a cup of coffee or a candy bar? Sure, those are still options. But now you'll find a dizzying array of so-called energy drinks claiming to increase stamina, improve concentration and perk up your performance. But do they work? And are they safe?

Unlike sports drinks, which usually contain carbohydrates and electrolytes to aid exercise endurance, energy drinks boost stamina temporarily by relying on megadoses of sugar and caffeine—in some cases more than twice the caffeine of an espresso. That's precisely what may be their downside as well. Excessive caffeine can make you jittery and irritable, raise your blood pressure and trigger heart palpitations, dehydration and insomnia. And the ill effects of sugar—tooth decay and obesity, to name a few—are well documented.

Some drinks contain vitamins, amino acids and herbs like ginseng. Avoid drinks with ma huang or ephedra, an amphetamine-like stimulant that's been banned as a dietary supplement. You should avoid energy drinks if you are pregnant or have:

It stands to reason that if energy drinks rev you up, they should help you run a little longer, walk a little faster and step a little higher. And they very well may—but at a high price. When you combine the fluid loss from exercise with the diuretic effects of a highly caffeinated drink, you're setting yourself up for serious dehydration. A better preworkout energy boost: Sip some water and nosh on an apple or some peanut butter.

The bottom line: If you're healthy, an occasional energy drink is probably safe for you, but proper sleep, healthy foods and regular exercise will give you better and healthier long-term results.

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