Impurities are present in virtually every manufactured product, and in some cases, even though the product may be considered harmless, the impurity is not. Such was the case in the late 1980's when an epidemic of cases of eosinophilia-myalgia syndrome, including over 30 deaths, were blamed on a contaminant present in L-tryptophan (9), an amino acid supplement widely taken as a sleep aid. Creatine, and other such supplements, are not regulated by the FDA. No published investigation has been conducted on creatine to determine what impurities might be present in creatine supplements, and what their long term effect might be.
The bottom line is that no one can confidently state that prolonged creatine supplementation is safe, and its use would best be avoided until more data can be compiled. Prolonged administration is, in essence, an uncontrolled toxicity study and one which might yield harmful results. Is it worth the risk? Remember, it's your body!