Creatine and Diarrhoea: One Trial Split the Dose, and the Single 10 g Serving Was the Problem

"Creatine diarrhoea" draws about 3,700 US searches a month, plus around 800 for creatine nausea. Most answers repeat that creatine is safe, which is true on the large-scale data, and leave out the one study that actually tested what changes the risk. That study found it was not whether people took creatine. It was how much they took at once.

This page sets out that trial, what the largest safety analysis adds, the ingredients in some products that are not creatine, and what has never been measured. The overall side-effect record, including the FDA report database, is on creatine side effects; this page is about the gut.

The trial that split the dose

Ostojic and Ahmetovic (2008) randomised 59 top-level male footballers in a double-blind design to one of three regimens for 28 days:

Across all three groups the most frequent complaints were diarrhoea (39.0%), stomach upset (23.8%) and belching (16.9%). The split group did not differ from placebo in gastrointestinal complaints. The single-serving group had significantly more diarrhoea than both the split group and placebo (p < 0.05).

What it shows. The same 10 g a day caused about twice as much diarrhoea taken at once as taken in two halves. The difference was the size of the serving, not the daily total.

What it does not show.

• Why. The trial recorded symptoms and did not measure anything in the gut. Explanations that circulate (undissolved powder, water drawn into the bowel) are hypotheses this trial did not test.
• The placebo rate. Thirty-five per cent of footballers on placebo reported diarrhoea over 28 days. Symptoms were collected by survey, and that background rate is high. Any figure from this trial reads against it.
• Women, older adults, non-athletes. All 59 were young elite male athletes.
• Duration. It counts who had symptoms, not for how long.

It is one trial of 59 men, and it is the study this question keeps coming back to. It bears directly on loading phases, which run at 20 g a day; the protocols, and the 1996 biopsy study they come from, are on creatine loading phase.

What 685 trials add

The largest safety analysis (Kreider et al., J Int Soc Sports Nutr 2025, PMID 40198156) pooled side effects from 685 creatine trials (12,839 participants on creatine, 13,452 on placebo):

• Counting studies: gastrointestinal issues appeared in 4.9% of creatine study groups against 4.3% of placebo groups (p < 0.001).
• Counting people: 5.51% of creatine participants against 4.05% of placebo participants (p = 0.820, not significant).

A study-level count records whether a symptom appeared anywhere in a paper. A participant-level count records how many people had it. When they disagree, the second is the one about people. The trials averaged about 12.5 g a day for about 65 days, more than the 5 g on most labels. Disclosure: the first author chairs the scientific advisory board of a creatine manufacturer, and a co-author sits on another of its boards, both stated in the paper.

The two sources are consistent. Across hundreds of trials, gut complaints barely separate from placebo. The one trial that varied the serving found that a large single serving is where they appear.

Nausea and stomach upset

Stomach upset was the second most common complaint in the footballer trial, at 23.8% across all groups including placebo. The trial does not report it by group. No trial found in PubMed has made nausea its own endpoint by serving size, so whether the dose pattern holds for nausea is unknown. The 2025 analysis does not report nausea separately from its gastrointestinal category.

When it is not the creatine

Sugar alcohols in gummies. A gummy that says "zero sugar" usually gets its sweetness and bulk from polyols. One brand's creatine gummy label, read on this site, lists maltitol syrup, isomalt, maltodextrin and xylitol as its first four other ingredients, with 9 g of carbohydrate per serving (Bloom creatine). Polyols are known to have a laxative effect in quantity. US food rules make that explicit for one of them: a food whose foreseeable use may supply 50 g of sorbitol a day must carry the statement "Excess consumption may have a laxative effect" (21 CFR 184.1835). No rule of that kind covers maltitol, isomalt or xylitol, and no creatine gummy trial has measured gut symptoms against a powder. The gummy format's other problems are on creatine gummies.

Flavoured powders and blends. Flavoured creatine often adds maltodextrin, acids and sweeteners, and pre-workout blends add caffeine. A symptom on a blend cannot be pinned on the creatine in it. What the trials found when creatine and caffeine were taken together is on creatine and caffeine.

What it was mixed with. Creatine monohydrate dissolves at about 14 g per litre at room temperature (Jäger 2011), so 5 g needs about 357 mL to dissolve fully. Many label directions give less. Whether undissolved creatine matters for the gut has not been tested; the solubility figures by temperature are on mixing creatine into drinks.

The hydrochloride claim

Creatine hydrochloride is sold partly on the gut. One product's retail listing carries "No Side Effects" in its title, and one brand's comparison table says ordinary creatine "doesn't absorb well leading to GI discomfort" (Costco creatine, Naked creatine). A PubMed search on 2026-10-04 for creatine hydrochloride with gastrointestinal, diarrhoea or diarrhea returned no records. There is no published trial of the hydrochloride form's gut tolerance to compare with the monohydrate data above. The two forms' wider evidence is on creatine monohydrate vs HCl.

What is not established

• The mechanism. No study has measured why a large single serving produces more diarrhoea.
• A threshold. Only two serving sizes have been compared (5 g and 10 g). Nothing shows where between them, or above, the risk begins.
• Duration and resolution. No trial reports how long symptoms lasted.
• Populations beyond young male athletes for the serving-size question.
• Formats. No trial has compared gut symptoms on gummies, capsules or hydrochloride with monohydrate powder.

Diarrhoea that persists, comes with blood or fever, or leads to dehydration is a matter for a clinician, whatever supplement is involved. The wider safety record, including kidney markers, is on creatine side effects, and the timing question is on creatine timing.

Sources and dates

• Ostojic SM, Ahmetovic Z. Gastrointestinal distress after creatine supplementation in athletes: are side effects dose dependent? Res Sports Med 2008;16(1):15–22, PMID 18373286, abstract read 2026-10-04.
• Kreider RB, Gonzalez DE, Hines K, Gil A, Bonilla DA. Safety of creatine supplementation: analysis of the prevalence of reported side effects in clinical trials and adverse event reports. J Int Soc Sports Nutr 2025;22(sup1):2488937, PMID 40198156, abstract and disclosures read 2026-10-04.
• US Code of Federal Regulations, 21 CFR 184.1835 (sorbitol), paragraph (e), read 2026-10-04.
• PubMed, search "creatine hydrochloride" AND (gastrointestinal OR diarrhea OR diarrhoea), 2026-10-04: 0 records.
• Jäger R et al. Analysis of the efficacy, safety, and regulatory status of novel forms of creatine. Amino Acids 2011;40:1369–83, PMC3080578 (solubility).
• Bloom creatine gummy label, read from the brand's product image 2026-10-04 (see the Bloom analysis).

Figures are reported as the sources state them. Nothing here is a recommendation or an instruction.