In biology, the hormone called PYY 3-36 plays a critical role in decreasing appetites, making us aware of fullness of our stomach. The injection of PYY in rats in an experiment revealed that they eat much less by 30% than usual, resulting in weight loss. This also applies to the human body. Therefore, researchers have thought that this could be used a very effective treatment for obesity, though it has not yet perfectly admitted as a practical drug.
The hormone that has the opposite role is known as ghrelin - instead of inhibiting the desire for food, it increases appetite.