What are some reason why a character might be prescribed 200 mg of thorazine?
...from a medical prospective.
Thorazine is used for the treatment of schizophrenia (severe disruptions in thought and perception). It is also prescribed for the short-term treatment of severe behavioral disorders in children, including explosive hyperactivity and combativeness; and for the hyperenergetic phase of manic-depressive illness (severely exaggerated moods).
Thorazine is also used to control nausea and vomiting, and to relieve restlessness and apprehension since surgery. It is used as an aid in the treatment of tetanus, and is prescribed for uncontrollable hiccups and acute intermittent porphyria (attacks of severe abdominal pain sometimes accompany by psychiatric disturbances, cramps in the arms and legs, and muscle weakness).
It's an anti-psychotic, for bipolar disorder or schizophrenia.