What it is
Creatine is a compound your liver, kidneys, and pancreas already make from amino acids. It's stored mostly in skeletal muscle as phosphocreatine, which is what your muscles burn through in the first 10 seconds of a hard effort, before the slower energy systems catch up.
Supplementing raises muscle creatine stores by roughly 20 to 40 percent above baseline. That is the whole mechanism. Everything else follows from it.
It is not a stimulant, it does not touch cortisol or testosterone in any meaningful way, and it is not a banned substance in any major sport. It is, functionally, a way to top up a fuel tank your body already uses.
What the evidence shows
Creatine monohydrate has been studied in well over 500 trials, which is unusual for a supplement. The consistent finding: it increases strength and power output in resistance training, typically adding a few extra reps per set once stores are saturated, and it modestly increases lean mass over 8 to 12 weeks largely through increased training capacity and water retained inside muscle cells (not bloat).
More recent trials in older adults show a second use case: paired with resistance training, creatine helps preserve muscle mass and strength during aging better than resistance training alone. There's also early but growing interest in cognitive effects, particularly under sleep deprivation, though the doses tested there are higher than the standard sports dose and the research is newer.
How to actually take it
You don't need a loading phase. Loading (20g/day for 5-7 days) saturates muscle stores faster, but 3-5g daily gets you to the same saturation point in about 3-4 weeks with no gut discomfort. Take it any time of day, with or without food, it doesn't matter, timing around workouts makes no measurable difference in the trial data.
Buy plain creatine monohydrate. Micronized versions dissolve a bit better in water but work identically. Ignore 'creatine HCl' and buffered variants marketed as superior, the trial evidence doesn't support a meaningful edge over monohydrate, and monohydrate is the version almost every positive trial actually used.
Side effects and who should be careful
The most common complaint is water retention, typically 1-2kg in the first few weeks, which is intracellular and not the same as fat gain or puffiness. Reports of kidney damage in healthy people have not held up under controlled study; the caution applies specifically to people with pre-existing kidney disease, who should talk to a doctor before starting.
Mild bloating or stomach upset happens in a minority of users, usually when taking it in one large dose on an empty stomach. Splitting it or taking it with food resolves this in most cases.