Reports :: Counting Number Of Distinct Records In Report?

I'm creating a report for an imaginary "medical clinic's database", the intended function of which is described as follows: "Create a statistic that shows the total number of distinct drugs prescribed to a patient."

Where I'm at:

I've created a query called UniqueDrugs containing drug and patient info. The SQL is:

Quote:

SELECT DISTINCT Drugs.Drug_Name, Patients.[Patient _ID]
FROM Patients INNER JOIN (Drugs INNER JOIN Prescriptions ON Drugs.[Drug_ID] = Prescriptions.[Drug_ID]) ON Patients.[Patient _ID] = Prescriptions.[Patient_ID]
GROUP BY Drugs.Drug_Name, Patients.[Patient _ID]
ORDER BY Patients.[Patient _ID];

The results of this query seem to be what I need. All I need is for the count formula that I use to return the number of distinct drug names there are that are related to a patient's ID.

I've created a report and I've put the following formula into a text box:

=DCount("[Drug_Name]","[UniqueDrugs]","[Patient_ID]=[PID]")

Where PID is the name of another text box in the report that contains the Patient ID that I want to compare to the drug names returned by the query.

However, this *always* results in #Error, no matter how I change the formula.I have been led to believe that syntax is not the issue, as the following formula worked as intended for a different task:

=DCount("[Prescription_Date]","[UniqueYearQuery]","[Patient_ID]=[PID]")

How do I use a DCount to return the number of different drugs that have been prescribed to a patient?

If it did not require the number of different drugs, I would simply use the following formula, which works just fine:

=Count(Drug_Name)