instruction: until 31 months ago, what was the first drug patient 1704 was prescribed via the po/ng route?
response: 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 LIMIT 1