what was the name of the prescription drug that patient 18841 was prescribed last until 11/2105?
select prescriptions.drug from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 18841 ) and strftime('%y-%m',prescriptions.startdate) <= '2105-11' order by prescriptions.startdate desc limit 1