Typically the weight gain associated with antidepressants and antipsychotics is related to their antihistaminic properties (yes allergy meds cause this too!). The antihistamine effect causes an increased appetite. This coupled with the patient feeling better so having a desire to eat can push the balance to a large weight gain as when someone has depression, they may not feel like eating.
Others may affect water balance and cause water retention by holding sodium in the body (like prednisone).
Some can slow the metabolism like antithyroid type medications.
Some affect insulin levels and can store more as fat by affecting the glucose balance system in the liver (think HIV drugs, diabetes medications, etc).
I know they are working on drugs to make people feel full so it makes sense that some drugs could even prevent that hormone system from working.
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