A medical alert can be defined by going to Content -> Custom Fields -> Add New. Use the below image as a reference to creating your medical alert field:

OR, existing custom fields already linked to the patient record can be marked as an alert field.

You will see the medical alert when you click into the patient record from the patient list.

You will also see it on the Information tab on the patient record, see below:

Alert fields will carry through to treatment notes and prescription.