A combination diet pill to help obese people lose weight has been found to be effective but hard to tolerate, researchers say.

A combination diet pill, which may soon become available in Australia, helps obese people shed the kilos and maintain the loss, new research suggests.

But the bad news is that the medication has some unpleasant side-effects, according to the study published in the Medical Journal of Australia.

The single-pill medication is a combination of phentermine, an appetite suppressant, and epilepsy drug topiramate.

The drug was approved in the US for use by obese adults in 2012.

A team of researchers at the Austin Hospital in Melbourne tested the drug on 103 patients.

Forty-one of 61 who stopped taking the pills, said they ceased because of side effects.

These included depression, dry mouth and tingling, prickling or burning skin sensations.

Mean weight for the group fell by 13 kilograms during the first phase of the treatment where patients ate a very low-energy diet.

For the 30 patients who completed the 22-month trial, their average weight fell by 6.7kg from the end of the low calorie diet phase to the time of their the last observation.

Lead researcher Professor Joseph Proietto said phentermine-topiramate therapy was not well tolerated but was effective in helping people maintain their weight once they'd shed the kilos.

"The purpose of our paper was to describe our experience with using the combination at some maximal doses to help people maintain weight after they've lost it," said Prof Proietto.

"Not to lose weight, you don't need drugs to lose weight.

"You need drugs to maintain it because of the biological fact that after people lose weight they get very hungry."

This single-pill drug has not yet been approved by the Therapeutic Goods Administration for weight loss in Australia.

But both phentermine and topiramate are available in Australia and GPs prescribe these drugs for obese adults "off label", a practice where doctors use a drug for a purpose that is different from the one approved by the TGA, Prof Proietto said.

The US Food and Drug Administration approved the combination therapy for weight loss in 2012, meaning an application to the TGA could be expected soon, Prof Proietto said.

However, Prof Proietto said there were advantages to having the drugs available separately in Australia.

"While we have to use them off label, the advantage of having separate drugs is that we can increase the dose up and down individually, so we can adapt it to each individual patient.

"There are some people that get side effects with one, but not the other, and vice versa."

As many as 11 million Australian adults, 63 per cent of the population, were overweight or obese in the 2011-12 financial year, according to the Australian Bureau of Statistics.

Meanwhile, another study published in the Medical Journal of Australia, found lap-band surgery was just as successful for those who received the treatment free in a public hospital, as it was for those who pay thousands of dollars to access it in a private hospital.

"There has been doubt about the motivation of obese patients whose surgery is fully subsidised," the authors wrote.

"But this study supported the hypothesis that patients reliant on public health care maintain sufficient intrinsic motivation to lose weight."