before it was labeled as an "antipsychotic," Thorazine was used in low doses in pre-operative sedation cocktails (using thorazine meant lower doses of barbiturates, which decreased the risks of surgery considerably...it also reduces nausea, so that's a big plus), and it was also used as a night time sedative.

based on its effects in non-psych indications, doctors in France started using it here and there in psych patients in hospitals...then they did more in-depth studies...and then Thorazine became the 1st popular, widely available major tranquilizer/antipsychotic.

it is worth noting that resperine and raowulfia alkaloids (probably not spelled right) were around then, too. those are from an Indian plant long used to soothe the distressed. thorazine was far more profitable.