Creatine has been a go-to supplement for bodybuilders and gym rats for decades. This amino acid, usually taken as a powder mixed with water or juice, bulks up muscles and helps athletes exercise longer and harder by ramping up production of adenosine triphosphate, or ATP, which supplies extra fuel. But is it safe? While there have been a rash of recent, worrying reports about the dangers of energy drinks (many of them contain creatine), for the most part it's safe. "Creatine wouldn't have survived on the market for 20 years if it had dangerous side effects," says Stu Phillips, a kinesiologist and outside expert for the Gatorade Sports Science Institute. "There's not a whole lot of evidence for any serious negatives." One caveat: Don't take creatine if you have a kidney condition, says Phillips.

It's efficacy, however, isn't as guaranteed as its safety. After about a week on the supplement, you should notice a slight weight gain and that your muscles get bigger. "Creatine pulls water into the muscles, so they swell pretty quickly," says Phillips. If you don't see these changes after week one, you probably never will. "You either respond to creatine or you don't," he says. "For 15 to 20 percent of people, their muscles just don't absorb it, so if you don't gain weight right away, creatine may not work for you."

In general, vegetarians tend to benefit the most from supplementation. While carnivores get natural creatine from meat and fish, vegetarians don't, so they may respond better. Furthermore, "there seem to be diminishing returns the longer you take creatine," says Phillips says. "There's not a lot of science to support this, but that's the general consensus among users."

Not all creatine sources are equal either. If you want to try creatine, first find a quality powder or pill. "Only buy creatine from a reputable manufacturer and reputable supplement store," says Dr. Brian Quebbemann, founder of The N.E.W. Program in Newport Beach, California. "What matters is the purity and concentration, so read the label. If a supplement has a low concentration of pure creatine, or creatine monohydrate, you'd need to ingest more to get the same benefit."

Once you've picked a product, start a five- or six-day "loading" phase. "Take 20 to 30 grams per day, split into two or three doses and spaced out evenly," says Phillips. Next comes the "maintenance" phase, where you taper back to one 4- to 5-gram dose per day, taken immediately after your workout. "Muscles take up creatine more effectively after exercise because they are spongy," he says. And always take creatine with a simple carbohydrate – it is absorbed more rapidly and incorporated into muscles better when insulin levels are high. If you do use powder, mix it with juice, or eat something sugary with a pill. But don't buy a premade creatine-carb combo, Quebbemann warns, because you'll get less creatine for your buck.