Group by with insensitive order

Suppose I'm doing a group by query like the following:

SELECT drug1, drug2, AVG(response)
FROM data
GROUP BY drug1, drug2

The problem is that the same drug may appear sometimes as drug1 and sometimes as drug2.  So, for example, the
combination"aspirin, acetaminophen" may also appear as "acetaminophen, aspirin" and I want these grouped together in
thequery.  Is there a simple way to do this? 

--Aram Fingal