Creatine is the most well-studied supplement of all time. I get more questions about this supplement than just about any other. I’m glad people are asking about it, because I recommend it to a lot of people. It is generally safe for everyone. It can in very few cases cause some stomach bloating or digestive distress. If you have kidney dysfunction, then you should consult your doctor before starting up on creatine (and supplemental protein for that matter).
People that take creatine can absolutely improve athletic performance and increase muscle mass. People also use creatine to treat certain brain disorders, neuromuscular conditions, congestive heart failure, along with some other conditions.
This is one of the best scientifically backed supplements, and has very clear implications in the strength and conditioning world.
A commonly discussed drawback is the retention of water weight and weight gain from taking creatine. This is likely blown out of proportion. How this works is that by supplementing with creatine, you saturate your muscle tissue with what is already naturally there, but in lower quantities.
When your muscle has more creatine in it, more water flows into your muscle cells. So the weight you would gain would all go to your muscles, producing a more full-muscle look. This is not FAT gain. The reality is at most 5 pounds of water weight gain in the muscles. That would only happen for a well-muscled individual that weighs a lot, to begin with.
A more realistic amount is probably .5-2lbs of water weight gain for most people. And remember, this is a good kind of weight gain in the muscle. Plus you get a performance and strength benefit from it that can make your training more impactful. It will also go away if you stop taking creatine.