how much magnesium sulfate, for the first time since 12/2105, was prescribed to patient 25733?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 25733 ) and prescriptions.drug = 'magnesium sulfate' and strftime('%y-%m',prescriptions.startdate) >= '2105-12' order by prescriptions.startdate asc limit 1