I looked up some information from the Natural Medicines Comprehensive Database about the efficacy/safety/hazards of creatine. I am pretty sure this is a site you have to pay for (maybe not) but I get it through the medical school. Some snipits from that:
"Tell patients who are controlling their weight and engaging in strenuous exercise and/or exercising in hot environments to avoid creatine supplementation "
"Creatine typically causes a weight gain of 0.5 to 1.6 kg that increases with prolonged supplementation (3997). This weight gain, most likely due to water retention, has been thought to also increase the risk for high blood pressure."
"POSSIBLY SAFE ...when used orally and appropriately. Creatine supplementation appears generally to be safe when used in appropriate doses in healthy adults. POSSIBLY UNSAFE ...when used orally in high doses. There is some concern that very high doses of creatine might adversely affect renal, hepatic, or cardiac function and have other adverse effects, such as hypertension; however, a clear association between high dose creatine and significant adverse effects has not yet been established."
There's a lot more information, but this is the basic idea. Our bodies do naturally make creatine. It is converted to creatinine and then filtered by our kidneys. Taking exogenous (supplement-form) creatine can decrease our abilities to make creatine endogenously (inside our bodies naturally). There are certain diseases that benefit from taking creatine supplementation, but if you are fairly healthy, I think it would be in your best interest to just use what your body makes on its own. If you are a vegetarian, it may be harder to make natural creatine, as fish and meat give us the building blocks with which to make it.
Please, if you are on creatine supplementation, let your doctor know when he/she does blood work/kidney function tests. This can alter the results and make it appear as if your kidneys aren't working efficiently.
-Becky