A new study has found the hormone retains fluid rather than builds muscle, although not all experts are convinced by the findings.
A team co-ordinated by Dr Anne Nelson of the Garvan Institute of Medical Research in Sydney reports its findings at a conference in Canada this week.
"Using high but safe doses, we were somewhat surprised to find that human growth hormone did not increase muscle mass or improve sports performance," says Nelson.
"We believe that if there had been a dramatic effect [on muscle mass] we would have seen it."
A total of 63 male and 33 female recreational athletes were recruited for the study. They were young and healthy and did 2-10 hours of exercise a week.
They were given either a placebo or a dose of human growth hormone (hGH) four times the body's normal level. The men were also given testosterone or a combination of testosterone and hGH.
After two months the study participants were tested for muscle mass, strength, power and endurance.
While the results showed that human growth hormone increased body mass, this was not mainly because of increased muscle mass.
"It put on fluid rather than muscle," says Nelson.
The team could also find no benefits of hGH to sports performance.
While hGH did not have any effects on its own, when taken with testosterone, it did increase muscle mass, says Nelson.
And it also increased one aspect of performance called sprint power, measured by how fast people could cycle on an exercise bike in short period of time.
Nelson speculates there is a synergistic effect between the two hormones, which act through different mechanisms.
The team also evaluated the effectiveness of a new hGH test they are helping to develop and found it could pick up the hormone several weeks after it was taken.
The researchers found the test was more sensitive when hGH and testosterone were taken together.
"My final words would be any one thinking of taking growth hormone should really think very carefully about it," says Nelson.
She says that as well as causing fluid retention, high levels of hGH taken long term can lead to cardiovascular problems and diabetes.
Sports physician Dr Peter Larkins says he hopes the findings are supported by future studies but for now he does not think they will stop the cheats.
"It's unlikely that people are going to be deterred because one study says we don't think [hGH] helps all that much," says Larkins, a medical consultant to the Australian Institute of Sport.
He says people are influenced by powerful anecdotal evidence from the gym, body building and weightlifting worlds that taking five to 10 times the normal levels of hGH makes you leaner and boosts muscle.
The body shape achieved when fluid is retained is quite different to what people are reportedly seeing, says Larkins.
He says he also finds it hard to believe that fluid retention is the main effect of growth hormone given its normally function in the body is to boost protein synthesis.
Larkins would like to see the study repeated using conditions that are closer to those in the field.
But Nelson says there are ethical restrictions on using higher doses and elite athletes.
"We had to be very careful that we didn't have anyone in the study who would be subject to drug testing, or in a situation where taking these drugs was banned," she says.
One interesting finding from the double-blinded study is that there was a very strong placebo effect.
"Half of those who were on placebo actually thought they were taking active treatment," says Nelson.
"In men 80% of the placebo group thought they were on active treatment."