Creatine is one of the most extensively studied supplements in sports nutrition, and its side effects are generally mild and infrequent. Across 684 randomized controlled trials, only 13.6% recorded even a single participant reporting a side effect from creatine, while 86.4% of trials reported none at all. The most common complaints are digestive issues and temporary water weight gain, with serious adverse effects being rare in healthy individuals.
Digestive Issues Are the Most Common Side Effect
Gastrointestinal symptoms top the list. In clinical trials, about 10% reported at least one digestive complaint in the creatine group. These include bloating, stomach discomfort, nausea, diarrhea, and occasionally constipation. In one 28-day trial, roughly 79% of all participants reported at least some undesired gut symptoms, with bloating and stomach discomfort being the most frequent.
Dose matters here. A loading phase, where you take around 20 grams per day for 5 to 14 days, is more likely to trigger digestive trouble than a standard 5-gram daily dose. In a head-to-head comparison, participants taking the loading dose reported more frequent and more severe GI symptoms than those taking 5 grams daily, though both groups had some complaints. If your stomach is sensitive, skipping the loading phase entirely and sticking with 3 to 5 grams per day is a simple way to reduce the risk. It just takes longer (about 3 to 4 weeks) to fully saturate your muscles.
Water Weight Gain in the First Week
Creatine is osmotically active, meaning it pulls water into your muscle cells. When creatine concentrations rise inside the muscle, water follows to maintain balance. This causes a temporary bump in body weight, typically 1 to 2% of your total weight during a loading phase. For a 170-pound person, that’s roughly 2 to 3.5 pounds. The International Society of Sports Nutrition notes that muscles can temporarily retain up to 1 liter of water during loading.
This initial weight gain is water, not fat. Some people feel puffy or bloated during this phase. Over the following weeks, as you continue training with creatine, any additional weight gain tends to come from actual increases in muscle mass and strength rather than water retention. Your scale weight may stay elevated or keep climbing, but your waist circumference may not change, or you may even look leaner.
Side Effects in Women
A systematic review and meta-analysis focused specifically on women found no significant difference in adverse events between creatine and placebo groups. Gastrointestinal symptoms were similar regardless of dosing regimen. Perhaps most notably, the review found no significant change in body weight in women taking creatine, contrasting with the water-weight gain more commonly reported in men. This is relevant because concern about weight gain is a frequently cited reason women avoid the supplement. The data suggests that concern is largely unfounded.
Creatine and Your Kidneys
This is probably the most persistent worry around creatine. The concern makes intuitive sense: creatine is processed by the kidneys, and supplementing increases creatinine (a waste product that doctors use to estimate kidney function) in the blood. Higher creatinine on a lab test can look alarming, but in a creatine user, it reflects increased creatine turnover rather than kidney damage.
A clinical trial that gave healthy men high-dose creatine for three months found no signs of kidney dysfunction. Kidney filtration markers actually improved over the study period, though the researchers attributed that partly to the aerobic exercise component. Across the broader body of evidence, including the 684-trial review, creatine does not increase the risk of renal side effects compared to placebo, even at higher doses or longer durations. The Mayo Clinic considers creatine likely safe for up to five years at recommended doses.
One important caveat: most of this research has been conducted in people with healthy kidneys. If you have pre-existing kidney disease, the safety profile may be different.
Does Creatine Cause Hair Loss?
This concern traces back to a single 2009 study of college rugby players that found a 56% increase in DHT (dihydrotestosterone) after seven days of a creatine loading phase. DHT is a byproduct of testosterone that can shrink hair follicles and contribute to pattern baldness. The finding spread widely online and stuck.
The problem is that no other study has replicated those results. Twelve additional studies have examined creatine’s effect on testosterone, and none found significant hormonal increases. The Cleveland Clinic’s summary is straightforward: no conclusive evidence suggests that creatine increases testosterone or causes hair loss. If you’re genetically predisposed to male-pattern baldness, it’s understandable to be cautious, but the current science doesn’t support a meaningful link.
Dehydration and Muscle Cramps
Because creatine draws water into muscle cells, a common assumption is that it dehydrates the rest of your body and makes you more prone to cramps, especially during exercise in the heat. A systematic review with meta-analyses looked at 10 controlled trials of athletes exercising in hot conditions and found no evidence for this. Body temperature regulation, hydration markers, plasma volume, and sweat losses were all unaffected by creatine supplementation. The incidence of cramping in creatine users was actually lower than or proportional to that of non-users.
Staying well-hydrated is still good practice when taking creatine, simply because your muscles are holding more water. But the supplement itself does not appear to increase your risk of heat illness or dehydration.
Less Common Side Effects
Beyond digestive complaints, clinical trials have occasionally reported:
- Musculoskeletal symptoms (muscle cramps or discomfort) in about 2.9% of trials
- Neurological or vestibular symptoms (headaches, dizziness, lightheadedness) in about 2.5% of trials
- Sleep, fatigue, or appetite changes in about 1.5% of trials
These rates are low, and importantly, placebo groups in these same trials often reported similar or even higher frequencies of the same complaints. That suggests many of these symptoms may not be caused by creatine at all.
Medication Interactions
Creatine has known interactions with six medications. Three are classified as moderate interactions: cimetidine (a heartburn drug), probenecid (used for gout), and trimethoprim (an antibiotic). These drugs can affect how your kidneys handle creatine, potentially increasing its concentration in the body. If you take any prescription medications, particularly ones that affect kidney function, checking for interactions before starting creatine is a reasonable step.
Dose and Duration Don’t Increase Risk
One of the more reassuring findings from the large 684-trial review is that neither higher doses nor longer supplementation periods meaningfully increased the likelihood of side effects. While statistical associations existed between dose and side effect reporting, the effect sizes were uniformly small, and the symptoms remained mild and nonspecific. Creatine monohydrate appears to be well-tolerated across a wide range of dosing protocols, including in studies lasting several months.