what was the drug that patient 21110 had been prescribed two times a month before.
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 = 21110 ) and datetime(prescriptions.startdate,'start of month') = datetime(current_time,'start of month','-1 month') group by prescriptions.drug ) as t1 where t1.c1 = 2