Creatine side effects: what is real and what is myth.

The safety record is unusually deep. Hundreds of randomised trials, continuous use for up to five years in healthy adults, and a recent analysis of more than 25,000 reported cases that found doses above 10 g a day taken for years to be safe. What follows is the honest list of what does happen.

The real side effects

Water in muscle

Creatine is osmotic: water follows it into the muscle fibre. Pooled data put the average weight change at under a kilogram over six weeks, most of it lean tissue, but loading can add more in days and a minority of people gain several kilograms quickly. The water is inside muscle cells rather than under the skin, so it reads as fuller muscles, not puffiness, and it settles within a few weeks. It reverses within about a month if you stop. If you compete in a weight-class sport, factor it in.

Stomach upset

Bloating, cramps or loose stools affect a minority, almost always during loading or after a large dose on an empty stomach. It is dose-dependent: mis-measuring a heaped tablespoon as a scoop and taking 30–40 g a day reliably produces diarrhoea within a fortnight. Keeping single doses to 5 g or less, taking creatine with food, splitting doses, and skipping loading resolves it for nearly everyone.

Feeling light-headed or wired

Some people report a jittery or dizzy feeling after a big dose on an empty stomach, especially when dehydrated. One proposed explanation involves creatine sparing the body's methyl groups, which then go into making adrenaline-like compounds; it is speculative. The fix is not: take it with food, or split it.

Blood tests: creatine, creatinine and creatine kinase

Three similar-sounding things cause real confusion at the GP surgery.

• Creatinine is the breakdown product of creatine and the marker doctors use to estimate kidney function. Supplementing raises it slightly, which can make estimated kidney function (eGFR) read lower than it is. This is a false alarm that resolves on stopping, and it is also raised by high-meat diets, dehydration and hard training.
• Creatine kinase is an enzyme that leaks from damaged muscle. It rises after hard training and is used as a marker of overtraining. It has nothing to do with supplementation.
• Creatine itself is not routinely measured.

Tell your GP you take creatine before any blood test so the results are read correctly, and avoid a hard session the day before.

Kidneys and liver

The most persistent worry, and the most thoroughly studied. Randomised trials lasting months to years, in healthy adults taking up to 30 g a day, have found no adverse effect on kidney or liver function. Sports-nutrition bodies conclude that creatine does not damage the kidneys of healthy people.

The exception is pre-existing kidney disease. There, the evidence is thin and caution is the sensible default. If you have chronic kidney disease, a single kidney, or a family history of kidney problems, discuss creatine with your GP or nephrologist before starting.

Hair loss

The claim traces to one 2009 study of 20 rugby players in which a week of loading at 20–25 g a day raised levels of DHT, a hormone linked to male-pattern baldness. The study did not measure hair, the DHT stayed within the normal range, testosterone did not change, and it has never been replicated. A later trial giving 5 g a day for several weeks to young men found no thinning and no follicle loss. On current evidence, creatine does not cause hair loss.

Dehydration and cramps

Because creatine pulls water into muscle, it was once assumed to leave the rest of the body short and to cause cramps, especially in the heat. Studies in athletes training in hot conditions found the opposite: creatine users had the same or better hydration and fewer cramps and heat-related problems. Creatine super-hydrates muscle rather than draining anything. Drink normally; there is no need to force extra water and no reason to stop in summer.

What changes at higher doses

Most of the safety data is at 3–5 g. Doses of 10 g a day have years of data behind them. Doses of 20 g or more a day are well tolerated for weeks, but their long-term effects, particularly on the brain, have not been studied. At higher doses expect more water retention, more stomach sensitivity, and a stronger case for splitting the dose. Higher intakes are for specific goals and short periods, discussed on the brain page, not for life.

Other myths, briefly

• "Creatine is a steroid." It is a compound found in meat and made by your own body, with no hormonal action. It is legal, sold in supermarkets and permitted in all sport.
• "You have to cycle off." No. Continuous daily use is how the long-term studies were run.
• "It makes you fat." The weight gain is water in muscle. Creatine contains no calories.
• "Caffeine cancels it out." Based on one small study. Later work suggests any interaction is minor; coffee is fine.
• "It causes acne." No good evidence. It has no hormonal action that would drive acne.
• "It is only for bodybuilders." The research now spans older adults, vegetarians, women, students and people recovering from injury.

Who should check with a doctor first

Creatine is suitable for most healthy adults, but speak to a GP, pharmacist or registered dietitian before starting if any of these apply:

• You have kidney disease, liver disease, or diabetes with kidney involvement.
• You take medication that affects the kidneys, such as high-dose anti-inflammatories, diuretics, or nephrotoxic drugs.
• You are pregnant or breastfeeding. Human trials are only now beginning; early signs are reassuring but the data does not yet exist, so most clinicians advise avoiding it.
• You are under 18. Reviews describe creatine as safe at normal doses in adolescents, but UK guidance recommends professional supervision for young athletes.
• You have bipolar disorder. A small number of case reports have linked creatine to manic episodes.

For everyone else, the practical summary is: 3 to 5 g of monohydrate a day, with food if your stomach is sensitive, drink normally, expect a little water in your muscles, and tell your GP before a blood test. That is the whole risk profile. The dosing guide covers the rest.

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