SELECT DISTINCT On Multiple Columns

I've read so many different pages on this topic but I can't seem to get my query the way it needs to be. Here's the query:

select admitnbr, lastname||', '||firstname||' '||finitial, hphone, mobile, wphone, med_rec, dob
from patients join schedule using (key_patien)
join adtmirro using (key_patien)
where appt_state = 'ON HOLD'

Because patients in my database can have multiple appointments "on hold" there are duplicates in the results. I only need 1 record per patient in order to forward this information into an automated dialer to contact that patient. I do NOT want to call the patient over and over again. Once will suffice. I'm trying to make a distinction on the column 'med_rec'. One row per 'med_rec' will be awesome but I can't find a way to create a distinct on that column.