It also has other health benefits, including lowering blood pressure and levels of harmful fatty molecules and sugar in the blood.
Researchers tested the two drugs phentermine and topiramate on almost 2,500 overweight and obese adults from 93 US centres.
Patients were split into groups who received a 56-week course of treatment with different doses of the drugs or a “dummy” placebo.
The results of the “Conquer” Phase III trial were published in an early online edition of The Lancet medical journal.
They showed that patients taking the drugs in higher doses lost 10.2kg, or 22lb, on average.
Overall, 70 per cent of patients in the higher dose group achieved five per cent weight loss. This compared with 21 per cent of those in the placebo group, and 62 per cent of those taking the lower dose.
The combination was more effective than orlistat, currently the only anti-obesity drug available on the NHS.
The scientists wrote: “Most importantly, weight loss achieved with phentermine and topiramate was sustained during 56 weeks with improvements in blood pressure, lipids (blood fats), glycaemia (blood sugar) and inflammatory markers.”