what is the total dose of dorzolamide 2%/timolol 0.5% ophth. that is prescribed to patient 2425 until 48 months ago?
select sum(prescriptions.dose_val_rx) from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 2425 ) and prescriptions.drug = 'dorzolamide 2%/timolol 0.5% ophth.' and datetime(prescriptions.startdate) <= datetime(current_time,'-48 month')