In the simplest terms, obesity is the product of a body's energy output being less than its energy input. But in reality, there's nothing simple about this complex and mysterious disease.

Obesity, which has skyrocketed in recent decades – now defining the body mass of over 40 percent of adult Americans – isn't just difficult for people to endure and scientists to understand. It's also incredibly hard to treat.

Beyond commitment to sustained lifestyle changes – healthy eating and exercise, effectively – there are really only two potential options that may help: bariatric surgery and weight-loss medications.

The former is invasive and carries various risks and complications. As for the drugs, they don't always work, and can have their own adverse effects too.

However, an experimental treatment recently trialled by scientists and detailed in a study published this week could open new doors for treating obesity patients with a weight-loss drug.

In the study, which involved almost 2,000 obese adults across 16 different countries, participants took a weekly dose of a drug called semaglutide, an existing medication already used in the treatment of type 2 diabetes.

A control group took only a placebo, in place of the medication. Both groups received a lifestyle intervention course designed to promote weight loss.

At the end of the trial, the participants who took the placebo lost a small but clinically insignificant amount of weight. But for those who took semaglutide, the effects were pronounced.

After 68 weeks of treatment with the drug – which suppresses appetite due to a variety of effects on the brain – participants taking semaglutide lost on average 14.9 percent of their body weight. And over 30 percent of the group lost more than 20 percent of their body weight.

Broadly speaking, this makes the drug up to twice as effective as existing medications for weight loss, the researchers say, approaching the kind of efficacy of surgical interventions.

"No other drug has come close to producing this level of weight loss – this really is a game-changer," says obesity researcher Rachel Batterham from University College London.

"For the first time, people can achieve through drugs what was only possible through weight-loss surgery."

In addition to losing weight, participants registered improvements in other areas, showing reductions in various cardiometabolic risk factors, and reporting quality of life improvements.

While the results are compelling, semaglutide dosage for anti-obesity effects does come with some drawbacks.

Mild-to-moderate effects were reported by many participants (in both the semaglutide and placebo groups), including nausea and diarrhoea. While the effects were temporary, they were enough for nearly 60 of participants to discontinue their treatment, compared with just five in the placebo group.

At present, the drug requires a weekly injection to work – whereas an oral form of the medicine would likely be preferred by patients.

More significantly, we don't yet have data on what happened to the participants after the drug regimen ceased at the end of the trial.

For at least one individual, however, who spoke to The New York Times, her weight began to creep up after the trial was over.

"While drugs like this may prove useful in the short term for obtaining rapid weight loss in severe obesity, they are not a magic bullet for preventing or treating less severe degrees of obesity," says nutritionist Tom Sanders, an emeritus professor at King's College London, who wasn't involved with the study.

"Public health measures that encourage behavioural changes such as regular physical activity and moderating dietary energy intake are still needed."

Nobody would deny the wisdom of that, but if further analysis of semaglutide turns out to be positive, we could also be looking at an important new pharmaceutical option to help combat obesity.

And that option might arrive sooner than we think.

The study, funded by pharmaceutical company Novo Nordisk – which sells semaglutide as an anti-diabetic medication – is now being tendered as evidence to international health regulatory authorities, in support of an application to market the drug as an obesity treatment.

The US FDA, along with its counterparts in the UK and Europe, is currently assessing the data.

The findings are reported in The New England Journal of Medicine.