[InSteady]

Creatine is produced endogenously and is believed to be a critical component of energy metabolism and expenditure, particularly in muscle tissue (including in the heart and skeletal muscle that keeps us alive).

As a supplement, it is extremely well researched (probably the most researched exercise supplement) and has been thoroughly demonstrated to have moderate enhancements to digestion, metabolism, and physical performance in all kinds of populations. It is also quite safe. Taking too much for too long can cause stomach discomfort, muscle cramps, dehydration, and in rare cases kidney damage.

The body produces about 1g per day naturally and humans who eat meat tend to get another gram or so through diet. Supplementing ~3g per day has been demonstrated to be extremely safe pretty much indefinitely (except possibly in people with liver or kidney disease, possibly other organ dysfunction).

Just like caffeine (and all manner of substances) if you have lots of pure powder and no clue about safe dosage, it can get dangerous very quickly.