Creatine is the most studied supplement in sport and still widely misunderstood. It isn't a stimulant or a steroid, and it doesn't build muscle directly. It buys you a few more seconds of maximal output, and everything else builds off that.
The mechanism uses the phosphocreatine system. ATP is the energy currency your muscle spends, but you only store a few seconds' worth. When you use ATP, it loses a phosphate group.
Phosphocreatine is also stored in muscle and it donates a phosphate to regenerate ATP almost instantly, which is what carries you through the first ten to fifteen seconds of maximal effort. Supplementation raises muscle creatine stores by roughly 20% on average, and phosphocreatine along with it.¹ More stored phosphate means faster ATP resynthesis between efforts, and quicker recovery.
Harris and colleagues, who first demonstrated this in 1992, found the increase was greatest in subjects who started with the smallest creatine stores — supplementation raised them towards the top of the normal range, while people already near that ceiling had less room to gain.¹ In some individuals the rise was as much as 50%. There is an upper limit to what muscle will hold, and how far you are from it before you start determines most of what you get. If you eat a lot of red meat, you're closer to it than a vegetarian is.
There are several loading protocols, a common recommendation involves: 20 g/day for six days to load the muscle, and 2-3 g/day to maintain it from there. Taking 3 g/day from the outset reaches the same concentration over about 28 days.² The endpoint is identical, but the initial loading gets you there quicker..
The performance increase is consistent: Branch pooled 100 studies and found the effect sizes small but real, and concentrated in a specific place: repeated bouts of work lasting 30 seconds or less.³ Single bouts benefited less than repeated ones, and running and swimming performance didn't improve at all.
One finding in that study is worth flagging if you strike or grapple: the effect was roughly twice as large for upper-body exercise as for lower-body work.³
Branch also found no difference between trained and untrained subjects, and none between men and women. Short bursts of effort, repeat sprints and heavy sets are where it helps. A 10 km run is not.
Two things worth knowing if you compete in sports with weight classes. Creatine increases total body water. Powers and colleagues loaded subjects at 25 g/day for a week, then 5 g/day for three more, and found body mass and total body water both rose.⁴
Expect one to two kilograms of body mass in the first weeks, most of it water rather than tissue. That's important when you have a target weight to hit.
On safety, the ISSN position stand is unambiguous: there is no compelling evidence that creatine monohydrate has any detrimental effect in otherwise healthy people, at intakes up to 30 g/day sustained for five years.⁵ That's six times a maintenance dose, for prolonged periods. The persistent claims about renal harm come from case reports in people with pre-existing disease, not from controlled trials, and from confusion with the similarly named creatinine molecule. If you have known renal impairment, talk to your doctor,
Practical position:
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3–5 g daily, indefinitely. Skip the loading phase unless you need saturation inside a week.²
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Take the monohydrate form. It's the most extensively studied form and the one the position stand backs; the alternatives cost more for no proven benefit.⁵
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Timing within the day is largely irrelevant. Consistency day to day is key.
References
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Harris RC, Söderlund K, Hultman E. Elevation of creatine in resting and exercised muscle of normal subjects by creatine supplementation. Clin Sci (Lond). 1992;83(3):367–74.
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Hultman E, Söderlund K, Timmons JA, Cederblad G, Greenhaff PL. Muscle creatine loading in men. J Appl Physiol. 1996;81(1):232–7.
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Branch JD. Effect of creatine supplementation on body composition and performance: a meta-analysis. Int J Sport Nutr Exerc Metab. 2003;13(2):198–226.
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Powers ME, Arnold BL, Weltman AL, Perrin DH, Mistry D, Kahler DM, et al. Creatine supplementation increases total body water without altering fluid distribution. J Athl Train. 2003;38(1):44–50.
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Kreider RB, Kalman DS, Antonio J, Ziegenfuss TN, Wildman R, Collins R, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr. 2017;14:18.