The text the you see below is from Wikipedia and I only copied relevant parts of it, please read the whole thing before using these to fatten yourself or others up, to know what to expect.
Olanzapine
Recent studies have established :
- that olanzapine and clozapine disturb the metabolism by making the body take preferentially its energy from fat (instead of privileging carbohydrates). Thus, levels of carbohydrates remaining high, the body would develop insulin resistance (reduction of insulin sensitivity).[32]
- that olanzapine promotes fat accumulation : due to disturbances in fat metabolism, rodents become fatter (but don't have their weight increasing at first). Being fatter, they do less exercise, burning less fat and gaining weight.[33]
Mirtazapine
...increased appetite and subsequent weight gain...
Ghrelin That is the hormone that makes you hungry, if that would be available in pills, it would be perfect for fattening up. Megestrol
A medication often used to make people eat that don't have any appetite otherwise. How it works with somebody that is already fat or gaining, well, what a dream thought!