until 31 months ago, what was the first drug patient 1704 was prescribed via the po/ng route?
select prescriptions.drug from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 1704 ) and prescriptions.route = 'po/ng' and datetime(prescriptions.startdate) <= datetime(current_time,'-31 month') order by prescriptions.startdate asc limit 1