There are also many studies examining the safety of creatine monohydrate. Here are a few examples:
Effects of long-term creatine supplementation on liver and kidney functions in American college football players. Int J Sport Nutr Exerc Metab. 2002 Dec;12(4):453-60.
Long-term oral creatine supplementation does not impair renal function in healthy athletes. Med Sci Sports Exerc. 1999 Aug;31(8):1108-10.
Long-term creatine supplementation does not significantly affect clinical markers of health in athletes. Mol Cell Biochem. 2003 Feb;244(1-2):95-104.
Dietary creatine supplementation does not affect some haematological indices, or indices of muscle damage and hepatic and renal function. Br J Sports Med. 2000 Aug;34(4):284-8
Effect of short-term creatine supplementation on renal responses in men. Eur J Appl Physiol Occup Physiol. 1997;76(6):566-7.
Creatine supplementation during college football training does not increase the incidence of cramping or injury. Mol Cell Biochem. 2003 Feb;244(1-2):83-8.
Acute creatine loading increases fat-free mass, but does not affect blood pressure, plasma creatinine, or CK activity in men and women. Med Sci Sports Exerc. 2000 Feb;32(2):291-6