how much metoprolol is first prescribed to patient 78086 until 6 months ago?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 78086 ) and prescriptions.drug = 'metoprolol' and datetime(prescriptions.startdate) <= datetime(current_time,'-6 month') order by prescriptions.startdate asc limit 1