This is where the waters get a little murky. Some negative side effects of creatine may include stomach pain, nausea, diarrhea, and muscle cramps. Reports of these seem to appear in a small percent of the population and vary between individuals. Claims of negative long term effects of creatine such as kidney, liver, or heart damage is more concerning but much less likely. To this day there is no scientific data that suggests creatine supplementation can harm these organs.