when was patient 73206 prescribed with a drug via im route for the first time since 05/2101?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 73206 ) and prescriptions.route = 'im' and strftime('%y-%m',prescriptions.startdate) >= '2101-05' order by prescriptions.startdate asc limit 1