It is not known whether the gain in weight is a result of more protein or simply from more water being stored. Isolated reports have linked creatine to problems with the liver and kidneys. The most common side effects are muscle tears, weight gain, dehydration and cramping, but doctors have not been able to directly prove the conditions are a result of creatine. In major league baseball, Baltimore's Brady Anderson has used creatine for five years, and such All-Stars as Mark McGwire of the St. Louis Cardinals, Gary Sheffield of the Florida Marlins, Ryan Klesko of the Atlanta Braves and Mike Piazza of the Los Angeles Dodgers are supporters. It definitely allows you to put on some weight.
Some people swear by it.
Others say it doesn't do anything. I think it's just like anything. A few years ago it was amino acids or a protein thing, then it was creatine. It will be something different in a year or two. Dante Bichette of the Colorado Rockies used creatine in his recovery from reconstructive knee surgery last winter.