Patients profiles database
You could have one table that tracks what a patient takes, and another that tracks which drugs interact with which:


patient_drugs
-------------
   patient_id (FK to main patient table)
   drug_id (FK to main drug table)


drug_interactions
-----------------
   drug_id (FK to main drug table)
   reacting_drug_id (FK to main drug table)
   interaction_description (text)


Then to find a patient's drug interaction you could maybe do this:

select *
from drug_interactions 
where drug_interactions.drug_id in (select drug_id
                                    from patient_drugs
                                    where patient_id = $PATIENT_ID)