The effects of Mysimba in reducing body weight have been shown in 4 main studies involving around 4,500 obese or overweight patients, in which Mysimba was compared with placebo (a dummy treatment). Patients in the studies were given the medicine as part of a weight loss programme involving counselling and advice on diet and exercise. The main measures of effectiveness were the percentage reduction in body weight over 28 or 56 weeks of treatment, and the proportion of patients who achieved at least a 5% weight reduction; the studies also looked at the number of patients who achieved at least a more stringent 10% reduction in weight, and the results were analysed using various methods to take account of the number of patients who did not complete the studies (around 50% over one year).

In three of the studies, the average weight loss in patients treated with Mysimba was around 3.7 to 5.7%, compared with 1.3 to 1.9% with placebo; the proportion of Mysimba-treated patients who achieved 5% weight loss ranged from 28 to 42% compared with 12 to 14% with placebo. About 13 to 22% of those taking Mysimba achieved at least a 10% reduction in weight, while 5 to 6% of placebo-treated patients did so.

In the other study, in which patient counselling was also more intensive, the overall weight loss was greater over the study period: 8.1% with Mysimba and 4.9% with placebo. Some 46% and 30% of patients given Mysimba achieved 5 and 10% weight reductions respectively, compared with 34% and 17% respectively with placebo.

The degree of improvement with Mysimba over placebo was similar using different methods of analysis, although the benefits were smallest with the most conservative methods that assumed patients who did not complete the study would not have seen any improvement. The treatment effect was more marked in patients who completed 56 weeks of treatment, or who had lost at least 5% of their original body weight by 4 months.