Obesity is a widespread problem
The drug, which has been developed by a team of researchers at Monash University in Australia, is to be tested throughout the rest of the year.
If trials are successful, it could be on the market in four years.
As the drug, named Advanced Obesity Drug 9604, works by speeding up the body's metabolism, it could theoretically help people lose weight without taking more exercise or cutting their calories.
However, the scientists see it only as a treatment to help people who are also using other methods to lose weight.
Obese men who took part in a trial lost an average of half a kilogram (1.1 lb) in a month after being injected with a single dose of the drug.
Major problem
Obesity is associated with health risks including heart disease, strokes, hypertension, angina and certain types of cancer.
Fat people are likely to die nine years younger than slim people.
Researcher Professor Frank Ng said: "We are hoping to give the drug to really obese people and reduce, to a certain extent, their fat so that they can become sufficiently mobile to begin the path towards improved health.
"Obesity used to be a condition associated with the trend toward old age but now we are seeing more and more children suffering obesity.
"This group has become our greatest concern but hopefully, using drugs like this one, we will be able to prevent them from becoming obese adults."
Sceptical
He said: "Past efforts which have worked via the metabolic pathway have either had side effects, or proven not to be effective.
"We need to be looking at changing the way people eat and encouraging them to be active and give up their sedentary lifestyles.
"This sort of thing, which offers the promise of a quick fix, just undermines attempts to give people sensible advice."
Latest figures show obesity now costs the UK economy over £2.5bn a year.
Levels of obesity in England have trebled since 1980 and one in five adults is now dangerously overweight.
Obesity also affects nearly one in 10 children under 11, according to a report from the National Audit Office (NAO).