Creatinine is a byproduct of the breakdown of creatine and phosphocreatine in the body. It is also a commonly used marker of kidney function.
Because supplementing with creatine can increase the blood levels of creatinine, there have been some concerns about a potential negative effect of creatine on kidney function. However, beyond a harmless increase in creatinine levels, scientific reviews on both the long- and short-term safety of supplemental creatine in people with healthy kidneys have consistently found no adverse effects on kidney function with a wide range of doses.[1][2][3][4][5][6][7][8] That said, although doses of >10 g/day have been found not to impair kidney function, fewer long-term trials have assessed the safety of such high chronic daily intakes.
Similar findings have been reported in trials looking at the effect of supplemental creatine in people with kidney disease, with trials reporting no detrimental effects on kidney function.[9][10][11] That said, long-term trials assessing the safety of supplemental creatine in people with kidney disease are lacking. Also, it’s worth keeping in mind that, because creatine can increase water retention, it could theoretically adversely affect individuals whose kidney disorder is being treated with diuretics, which cause water loss.
Overall, the available evidence suggests that short- and long-term supplementation with creatine is likely safe for people with healthy kidneys, and that short-term supplementation with creatine is likely safe for people with suboptimal kidney function. Less is known about the effects of long-term supplementation with creatine in people with suboptimal kidney function.
Also, it’s worth keeping in mind that, although elevated creatinine levels in response to supplementation with creatine are not indicative of kidney damage, creatine’s ability to raise creatinine levels could potentially mask underlying health issues.