how much phenylephrine is prescribed to patient 9983 first time until 36 months ago?
select prescriptions.dose_val_rx from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 9983 ) and prescriptions.drug = 'phenylephrine' and datetime(prescriptions.startdate) <= datetime(current_time,'-36 month') order by prescriptions.startdate asc limit 1