what is the amount of phenytoin sodium prescribed to patient 32168 totally until 47 months ago?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 32168 ) and prescriptions.drug = 'phenytoin sodium' and datetime(prescriptions.startdate) <= datetime(current_time,'-47 month')