Creatine is generally considered safe, but on rare occasions it can cause weight gain, nausea, vomiting, diarrhea, difficulties in breathing, fatigue, fever, kidney problems and headaches. It is not recommended for use by anyone with kidney or liver disease or diabetes, according to WebMD. The likelihood of side effects occurring increases when ingesting caffeine or other stimulants.
WebMD explains that creatine's negative side effects occur when it is orally taken in high doses. Because it causes the muscles to draw more water from the rest of the body, dehydration can occur. There is some evidence that combining creatine with ephedra, also known as Ma Huang, increases the chance of strokes, but no studies have been conducted examining the relationship between the two as of 2014.
Creatine has also been linked to irregular heartbeats and pigmented purpuric dermatosis, a skin condition, but the relationship has not been studied as of 2014.
Creatine is found in the muscles and can be obtained by eating meat and fish. Athletes looking to improve performance and vegetarians who need to supplement their protein take creatine supplements. People who already have high creatine levels typically see less benefits than those with low levels, since skeletal muscle cannot hold creatine past its saturation level, explains WebMD. WebMD also states that creatine has also been used to slow the progression of Parkinson's Disease.