Creatine supplementation is perhaps the most popular sports supplement, and when something gets to be as large as it is people tend to worry about every small possible side effect. When it comes to creatine there was initially concern about how it affected the kidneys because:
(1) Generally speaking, the kidneys are a major detoxifying organ that are going to interact with whatever you put in your body. Creatine is going to meet the kidneys eventually if it is managed to be absorbed from the intestines
(2) Creatine is known to be processed by the kidneys, and the metabolite (break down product) known as creatinine can be measured in urine
Ingesting creatine will increase the amount of creatinine in the urine, which makes sense since you're consuming something that is broken down and needs to be processed from the body. However, measuring creatinine in the urine used to be the gold standard of assessing kidney damage; does the increase in creatinine seen with creatine supplementation mean that the kidneys are being damaged or does it an observation error?
All current evidence shows that, for people with healthy kidneys, creatine does not impair their kidney health in any way. The evidence is collected in the [Human Effects Matrix using studies that measure other aspects of kidney health and see that they are essentially unchanged, meaning that creatinine is the only biomarker that is altered from creatine ingestion and it is considered to be harmless.
There is even evidence where in situations where the kidneys do not function optimally, such as this study in a man taking a large dose of creatine (20 grams) while only having one kidney functioning, there is still no damage seen. However, even if the evidence currently states you can take creatine with poor renal function you should still notify your doctor before taking supplements.
The only potential harm from creatine on the kidneys, which is currently theoretical (based on case studies), are in cases where somebody has a kidney disorder that is being treated with diuretics. In some medical conditions diuretics can help the patient and consuming anything that increases water retention can work against the drug; creatine, as an osmolyte, may very well increase water retention and work against diuretics.