DOCTOR ON CALL ASK THE EXPERT: What is creatine? Our rugby coach regularly warns us not to take it. I know some guys who do and it doesn't seem to be doing them any harm.

I'M NOT surprised some of your team-mates take creatine supplements. A survey of 674 high school athletes in the US found that 16 per cent used creatine in an effort to improve sports performance. Creatine users report less fatigue and more sustained performance and they say it helps high-intensity, short-duration exercise.

Creatine is a naturally occurring substance found in muscle. It plays a key role in supplying energy to our muscles.

In theory, raising creatine levels prolongs the activity of skeletal muscle.

The daily amount of creatine needed by the body is about two grammes for someone weighing 70 kilos.

A diet including fish and meat ensures an adequate daily intake.

The main reason your coach discourages the use of creatine is that there are no authoritative studies of the long-term effects of taking the supplement.

In addition, creatine is not available as an authorised medical product, which means that when you buy it over the internet or from a friend there is no guarantee the product does not contain other chemical substances or the actual dose is the same as that described on the label.

For example, there have been reports of creatine products containing the banned stimulant ephedrine.

The real value of creatine supplementation remains uncertain. Additional creatine has been shown to increase the creatine content of muscle by 20 per cent. Those who undertake a weight-training programme for the first time appear to benefit the most. Elite athletes whose diet contains adequate protein probably have the least to gain from additional creatine.

The protein needs of athletes depend on their weight and activity levels. The recommended daily allowance (RDA) for protein is 0.8 grammes per kilogram of body mass; a normal mixed diet delivers the required amount.*

Without good research it is hard to be precise about the short-term side effects of creatine.

However, regular users report tummy upset, dehydration and increased muscle cramping.

There is a suggestion that supplemental creatine increases your risk of muscle tears, such as pulled hamstrings.

Creatine increases body weight because it encourages water retention and there have been isolated reports of it causing kidney failure.

Until proper long-term research is done, I reckon you should take your coach's advice.

Dr Muiris Houston is Medical Correspondent ofThe Irish Times . Email your queries on sports medicine to sportsdoctor@irish-times.ie