what was the dexamethasone dose prescribed to patient 20166 last in their last hospital encounter?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 20166 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.drug = 'dexamethasone' order by prescriptions.startdate desc limit 1