Medical researchers in Sydney say they have developed a weight loss drug that could change the way the body uses fat.

Scientists from the Garvan Institute of Medical Research say the new drug can control weight gain in mice by stopping the body from receiving certain signals from the brain.

Traditional weight loss drugs try to stop the brain from sending hunger signals to the body.

But the head of the neuroscience program at the Garvan Institute, Professor Herbert Herzog, says these therapies tend to be ineffective.

"What we've found is that blocking one system that influences appetite and body weight regulation might not be enough to cause a significant change in reducing body weight," he said.

"If you take one signal away, others take over."

Patients who use these traditional weight therapies only lose about 5 per cent of their body weight.

The Garvan Institute researchers decided to reverse this approach and stop the body from receiving hunger signals from the brain.

Not only does the drug control appetite, it also encourages the body to burn more fat.

"The body can decide if it uses fatty acids or protein for generating energy. So if one can direct the body to use more fatty acids, which is stored in fat tissue, then the reduction of body weight might be greater," Professor Herzog said.

The drug has proven to be successful in mice. The researchers found that the mice lost body fat but their muscle mass stayed the same.

The Garvan Institute says it will probably be at least three years before the weight therapy will be ready for human trials.

But if it is successful, the drug could be helpful for those who need to lose a moderate amount of weight.

"It will definitely improve the effects of losing weight by a normal diet," Professor Herzog said.

"This might not be really suitable for extremely obese people, but for moderately obese people, it might be a real alternative."