Take the "drink lots of water" advice seriously. I took a creatine supplement for about a month when training for a race. I was running 5 days a week and felt only slightly dehydrated at the time. At the end of one run, I got the sensation of someone kicking me in the crotch and stabbing me in the lower back with a screw driver. It progressively got worse over the course of two hours, and I had to ask a friend to drive me to the ER. By the time we got to the emergency room, i was balled up in a fetal position and could barely respond to the doctor because of the intense pain. Lucky for me the seasoned old doctor could tell immediately what was happening. Apparently I developed a kidney stone from excess creatine in my system and not enough fluids. This was by far the worst pain I have ever felt in my life. So once again, drink lots of water! It's no joke.
BTW... If you start to get the feeling described above, grab a beer immediately and slam it. There's something in beer that helps break up and pass the kidney stone.