I used to be perfectly sure that creatine was fine to take. After all, it is natural and found in foods I eat daily. Now, I am not so sure. I met someone recently who threw me for a bit of a loop.
After talking with him for a short while, I asked what had happend to his arm, which had bad scars and pieces missing. He made the claim that using creatine had caused it. He claimed that it resulted in a blood clot and that many others have experieced similar and worse problems with creatine. He said that he only took it for four months and way below the recommended dosage.
Normally, when someone tells me about the dangers of supplements they seem to be talking out of their ass, but this guy had some pretty convincing evidence (just looking at his arm).