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Half were given the jab and half a placebo.
They completed surveys on their eating habits and appetite at the start of the trial and again after 20 weeks. They had a set breakfast, followed by an unlimited lunch five hours later.
Patients on the drug ate 1,736 calories at lunch, which was 940 fewer than those on the placebo.
Those given the jab lost 9.9 per cent of their body weight on average, compared with 0.4 per cent in those on the dummy drug.
Patients on the medication reported a smaller appetite and better hunger control during the study.
The jab is available at a dose of 1mg to treat type 2 diabetes but the trial used 2.4mg.