Change "Sig" field Textbox into a Drop-down list?

Title is Pretty self-explanatory. how do I turm the Sig textbox into a Drop-down list?
(In the add prescription tab)

I think you need the list of possible meds populated.
Are you using electronic prescribing?
Have you added a table of meds?
From other posts it looks as though once you do this you automatically get a drop down