what was the promethazine hcl dosage that patient 52456 was last prescribed during their first hospital encounter?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 52456 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and prescriptions.drug = 'promethazine hcl' order by prescriptions.startdate desc limit 1