Doctor Answers (3) on USING CREATINE
The many scientific studies of creatine published over the past five years have found no deleterious effects or medical risk. While there have been some isolated anecdotal reports of negative side effects, such as decreased sexual functioning, there appears to be no credible evidence challenging its safety.
One concern some researchers have is that with continuous supplementation the body might decline in its ability to produce creatine on its own. Some indirect evidence of this was found in a study with rats. Researchers found that chronic supplementation with creatine in rats down-regulated their natural production of creatine transporter protein.
Another issue concerns the other ingredients that accompany creatine in various supplement products. You should compare labels of competing brands carefully to determine whether you feel comfortable with the other ingredients.
Although no risks from creatine supplementation in humans have yet been found, we won't know about any long-term effects for some time to come because the record of research is relatively recent.