what's the name of the drug that patient 1417's been prescribed two times since 18 months ago?
select t1.drug from ( select prescriptions.drug, count( prescriptions.startdate ) as c1 from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 1417 ) and datetime(prescriptions.startdate) >= datetime(current_time,'-18 month') group by prescriptions.drug ) as t1 where t1.c1 = 2