*quick search*
There are quite a few studies around on PubMed (try
www.pubmed.com and search "creatine supplement" and perhaps add in "long term"). You can get the abstracts freely, though I'll admit it's more problematic to get the articles themselves.
I've just checked Medline and a few others I have access to from the hospital. There are a few reports studying cohorts for up to 5 years, none of which have managed to show any problems with liver or kidneys. Reports of problems have all (AFAIK) been isolated case reports, generally accepted to have been people who had pre-existing renal disease.
"Scientific publications on the undesirable effects of exogenous creatine are almost non-existent, but newspapers and the media do not hesitate to publish false information or wrong interpretations based on one individual case of an individual with nephropathy who, later on, consumed creatine monohydrate. In addition, we did not find any side-effects on renal function after short-term (five days), medium-term (nine weeks) or long-term (up to five years) creatine supplementation in small cohorts of athletes. Future prospects: We have no intention to validate or not the supplementation of exogenous creatine by athletes, but apparently there are no real deleterious effects on the body of healthy consumers. " - Quick quote from an abstract of a review article in "Science and Sports"
However, as
human points out, taking
anything in great quantity (normal foods included) for a long time is probably a bad idea. But it doesn't seem there's anything dangerous about creatine in itself (after all, creatine is naturally present in meat - albeit at only 4g per kilo)