A team led by Dr Kishore Gadde from Duke University Medical Centre, North Carolina, studied dual therapy with these two drugs in 2,487 overweight or obese adults.
The researchers compared two doses of the drugs with placebo in patients also given diet and lifestyle and metabolic risk reduction treatments.
After one year, patients assigned to placebo lost 1.4kg on average. Those given phentermine 7.5mg and topiramate 46mg lost 8.1kg and those given phentermine 15mg and topiramate 92mg lost 10.2kg.
The researchers concluded: 'The robust efficacy of phentermine plus topiramate shown in this trial compares favourably with the efficacy shown by existing and other emerging drugs.'
They said topiramate plus diet and exercise advice could be a valuable addition to the 'small arsenal of effective obesity treatments' available to family doctors.