until 5 months ago, how much terazosin has been prescribed to patient 64538 in total?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 64538 ) and prescriptions.drug = 'terazosin' and datetime(prescriptions.startdate) <= datetime(current_time,'-5 month')