The first six months you stay the weight you are now. We then introduce a single change: we put you on antipsychotic medication. Nothing else changes: you're eating the same type of food and in the same quantities.
You will gain weight.
Is that because thermodynamics is broken? No.
Is it because you're "undisciplined"? No.
Is it because the antipsychotic medication is full of delicious calories? No.
It's because your carbohydrate pathwathway has been altered, and you now process carbs differently to how you used to.