Considering creatine’s popularity as a sports supplement, it isn’t surprising that concerns have been raised about potential adverse effects — especially on the kidneys.
Why the kidneys?
To store the creatine you ingest, and the creatine your liver produces, your skeletal muscles,  your brain,  and other tissues transform it into phosphocreatine. And when they use their stores to recycle adenosine triphosphate (ATP), often called life’s energy currency, one of the byproducts is creatinine.
Your kidneys excrete this creatinine unchanged. If their function declines, so does creatinine clearance. Creatinine clearance can be estimated from a blood test or measured through a combination of blood test and 24-hour urine sample.
Creatinine blood levels are the most commonly used indicator of kidney function: it is assumed that if your levels are high, your kidneys haven’t done their job.
Your skeletal muscles make for >90% of your creatine stores,  so the more muscle you have, and the more you use it, the more creatinine you produce.   Also, greater animal protein intake has been associated with greater muscle mass   and higher creatinine excretion.  And yes, supplemental creatine can increase your blood levels of creatinine beyond the normal ranges;   but can we really conclude your kidneys are being damaged?
Concerns have been raised over creatine supplementation because it can increase creatinine levels beyond the normal range — a marker of kidney damage.
The evidence collected in our creatine Human Effect Matrix shows that creatine supplementation does not affect any measure of kidney health apart from creatinine levels. Moreover, scientific reviews on both the long- and short-term safety of supplemental creatine have consistently found no adverse effects on kidney function.        
Most healthy people can reap the performance benefits of creatine with just 3–5 g/day, but creatine nonresponders and people with high muscle mass may benefit from 10 g.  Doses >10 g/day have been found not to impair kidney function, but there are fewer long-term trials on such high doses. 
In healthy adults, doses ≤5 g/day are unlikely to increase creatinine levels significantly,    but higher doses might cause a false positive — an increase in creatinine that may be misinterpreted as a sign of kidney damage.    Most studies, however, have noted only a small increase in creatinine levels even with doses ≈20 g/day.   
Although taking creatine may increase creatinine levels, long- and short-term studies have found that creatine doses ≤10g/day don’t impair kidney health in people with healthy kidneys.
In people whose kidneys don’t function optimally, supplemental creatine seems to be safe, too.
One case study found that a man’s single kidney suffered no harm from 20 g/day for 5 days followed by 5 g/day for 30 days.  An RCT in type 2 diabetics with various stages of kidney disease showed no significant decline in kidney function from 5 g/day for 12 weeks.  Two clinical trials saw no detriment in people with kidney disease.  
However, studies in people with suboptimal kidney function are fewer than in healthy people, and they are short-term. People with kidney dysfunction, or at risk for developing kidney dysfunction (e.g., people with diabetes, high blood pressure, or family history of kidney disease; people over sixty; and non-Hispanic blacks), might wish to forgo creatine, or otherwise take only the lowest effective dose (3 g/day)  after talking to their doctor.
Finally, let’s mention that, since creatine can increase water retention, it could theoretically harm people whose kidney disorder is being treated with diuretics, which cause water loss. 
Limited short-term evidence indicates that people with suboptimal kidney function may safely supplement with creatine. However, long-term studies are lacking.
While creatine is unlikely to hurt your kidneys, its ability to raise creatinine levels may mask underlying issues. Consider having your creatinine levels tested before you start taking creatine, so as to get a baseline and check up on your kidney function. If you are already taking creatine yet plan to have your creatinine tested, cease supplementation 3 weeks prior to testing so as to prevent a false positive.
Short- and long-term creatine supplementation does not appear to impair kidney function in people with healthy kidneys.
Limited short-term evidence has found creatine to be safe for people with suboptimal kidney function, but caution is warranted.
When caused by supplemental creatine, elevated creatinine levels are not indicative of kidney damage. Creatine’s ability to raise creatinine levels may mask underlying issues, however, so if you plan to check up on your kidney function by having your creatinine levels tested, stop taking creatine at least 3 weeks before the test.
For more information on creatine, see our dedicated supplement page. If you are looking into creatine for fitness purposes, see our Muscle Gain & Exercise Performance Supplement Guide or our Fitness Guide (which contains our Muscle Gain & Exercise Performance Supplement Guide).
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