Do energy drinks really rev up your body and sharpen your mind? And what, exactly, are they even made of? To help you separate the science from the sales pitch, the authors of the new book, Eat This, Not That! The Best (and Worst!) Foods in America analyzed the claims and ingredients of five of the most popular potions on the market, and rated them from best to worst. All to answer the most important question of all: Are energy drinks safe—or should you can these beverages for good? Read on for our exclusive report.

5. 5-Hour Energy (2 fl oz)

• 4 calories
• 0 g sugar
• Exact caffeine content not provided by the company

The claim: “The two-ounce energy shot takes just seconds to drink and in minutes you’re feeling bright and alert. And that feeling lasts for hours.”

The truth: Sure, it’ll give you a jolt. That’s because it’s mainly caffeine—about the same amount that’s in one cup of coffee, according to label claims. (So somewhere between 65 to 135 mg of caffeine.) And turns out, the half-life of caffeine—the time it takes for half of the stimulant to be eliminated from your body—is about five hours. What’s more, the company touts that since the product doesn’t contain sugar, you won’t experience the sugar crash that comes a couple of hours after guzzling the sweet stuff. And that’s true, too. Of course, you could just grab a cup of unsweetened Joe for the same effect.

Is it safe? Downing a bottle should be no problem for a regular coffee drinker. Too much caffeine, however, could cause headaches, sleeplessness, nausea, hallucinations, and a spike in blood pressure. (Sodium can also spike your BP—be sure to avoid these saltiest foods in America.

4. Starbuck’s Double Shot Energy and Coffee (15 fl oz)

• 210 calories
• 26 g sugars
• 146 mg caffeine

The claim: “A powerful, great-tasting brew of B vitamins, guarana, ginseng, and natural proteins from milk. Charged up with coffee. That extra surge to keep you energized and alert.”

The truth: Most energy drinks laud their herbal supplements, but the science behind the add-ins is somewhat fuzzy. Ginseng, for example, won’t give you an energy blast, although it might boost your brainpower. For instance, Australian researchers found that people who swallowed 200 mg of the extract an hour before taking a cognitive test scored significantly better than when they skipped the supplement. And guarana’s benefit may simply be due to its caffeine content—a guarana seed contains 4 to 5 percent caffeine (about twice as much as a coffee bean). Fancy marketing ploys aside, the Double-Shot ultimately one-ups the competition by virtue of containing actual health-boosting coffee—a beverage that delivers disease-fighting antioxidants. Check out other functional food fixes here.

Is it safe? Ginseng has been shown to interact with certain medications, like the blood-thinner warfarin, potentially altering its effectiveness. And scientists at Florida’s Nova Southeastern University concluded that the amount of guarana found in most energy drinks isn’t large enough to cause any adverse side effects. However, there's still a question as to the safety of downing a few cans of the stuff in a brief time span.

3. Red Bull (8 oz)

• 110 calories
• 27 g sugars
• 76 mg caffeine

The claim: “With Taurine. Vitalizes body and mind.”

The truth: Caffeine certainly offers brain-boosting benefits, and the added slew of B-vitamins are conceivably helpful for a more efficient metabolism. Unfortunately, the sugar and taurine work to counteract those forces. A New Zealand study found that even the 27 grams of sugar in Red Bull is enough to completely inhibit your body’s ability to burn fat. (Check out the sugar-packed beverages on our list of the worst drinks in America. And taurine, an amino acid that works as a neurotransmitter, might act more like a sedative than a stimulant, according to researchers at Weill Cornell Medical College.