when was the first time patient 72107 had dextrose 50% prescribed on their first hospital visit?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 72107 and admissions.dischtime is not null order by admissions.admittime asc limit 1 ) and prescriptions.drug = 'dextrose 50%' order by prescriptions.startdate asc limit 1