when was the first time that patient 54494 was prescribed with pregabalin during their last hospital encounter?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 54494 and admissions.dischtime is not null order by admissions.admittime desc limit 1 ) and prescriptions.drug = 'pregabalin' order by prescriptions.startdate asc limit 1