Minor gastrointestinal upset was reported in some individuals during a long term study but it only occurred during the loading phase when a high dose of creatine monohydrate was taken. This could be avoided by lowering the dose during the loading phase or skipping the loading phase entirely. The same study also found that there was no difference in reported muscle injury, cramps, or any other side effects associated with creatine use which suggests that long term use does not lead to adverse health effects (13).
When investigating the long term effects of creatine on kidney function, there was no difference between creatine users and a control group (6,14). American college football players also showed no signs of kidney or liver damage during a long term study (10).